Semiconductor device, display device including the semiconductor device, display module including the display device, and electronic device including the semiconductor device, the display device, and the display module

ABSTRACT

To provide a semiconductor device including a planar transistor having an oxide semiconductor and a capacitor. In a semiconductor device, a transistor includes an oxide semiconductor film, a gate insulating film over the oxide semiconductor film, a gate electrode over the gate insulating film, a second insulating film over the gate electrode, a third insulating film over the second insulating film, and a source and a drain electrodes over the third insulating film; the source and the drain electrodes are electrically connected to the oxide semiconductor film; a capacitor includes a first and a second conductive films and the second insulating film; the first conductive film and the gate electrode are provided over the same surface; the second conductive film and the source and the drain electrodes are provided over the same surface; and the second insulating film is provided between the first and the second conductive films.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor film and a display deviceincluding the semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. In particular, one embodiment of the present inventionrelates to a semiconductor device, a display device, a light-emittingdevice, a power storage device, a storage device, a driving methodthereof, or a manufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a field-effect transistor (FET) or a thinfilm transistor (TFT)). Such transistors are applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). A semiconductor material typified bysilicon is widely known as a material for a semiconductor thin filmapplicable to a transistor. As another material, an oxide semiconductorhas been attracting attention.

For example, a technique in which a transistor is manufactured using anamorphous oxide containing In, Zn, Ga, Sn, and the like as an oxidesemiconductor is disclosed (see Patent Document 1). Furthermore, atechnique in which a transistor using an oxide thin film and aself-aligned top-gate structure is manufactured is disclosed (see PatentDocument 2).

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165529-   [Patent Document 2] Japanese Published Patent Application No.    2009-278115

SUMMARY OF THE INVENTION

As a transistor including an oxide semiconductor film, an invertedstaggered transistor (also referred to as a transistor having abottom-gate structure), a planar transistor (also referred to as atransistor having a top-gate structure), and the like are given. In thecase where a transistor including an oxide semiconductor film is usedfor a display device, an inverted staggered transistor is used moreoften than a planar transistor because a manufacturing process thereofis relatively simple and manufacturing cost thereof can be kept low.However, signal delay or the like is increased by parasitic capacitancethat exists between a gate electrode and source and drain electrodes ofan inverted staggered transistor and accordingly image quality of adisplay device degrades, which has posed a problem, as an increase inscreen size of a display device proceeds, or a display device isprovided with a higher resolution image (for example, a high-resolutiondisplay device typified by 4 k×2 k pixels (3840 pixels in the horizontaldirection and 2160 pixels in the perpendicular direction) or 8 k×4 kpixels (7680 pixels in the horizontal direction and 4320 pixels in theperpendicular direction)). Furthermore, as another problem, theoccupation area of an inverted staggered transistor is larger than thatof a planar transistor. Thus, with regard to a planar transistorincluding an oxide semiconductor film, development of a transistor whichhas a structure with stable semiconductor characteristics and highreliability and which is formed by a simple manufacturing process isdesired.

With the increase in the screen size or the resolution of the displaydevice, the structures of a transistor formed in a pixel of the displaydevice and a capacitor connected to the transistor become important. Thecapacitor functions as a storage capacitor for storing data written tothe pixel. Depending on the structure of the capacitor, there has been aproblem in that data written to the pixel cannot be stored and the imagequality of the display device is degraded.

In view of the foregoing problems, one object of one embodiment of thepresent invention is to provide a novel semiconductor device including atransistor having an oxide semiconductor. In particular, one object isto provide a semiconductor device including a planar type transistorhaving an oxide semiconductor. Another object is to provide asemiconductor device including a planar transistor having an oxidesemiconductor and a capacitor connected to the transistor. Anotherobject is to provide a semiconductor device including a transistorhaving an oxide semiconductor and having high on-state current. Anotherobject is to provide a semiconductor device including a transistorhaving an oxide semiconductor and having low off-state current. Anotherobject is to provide a semiconductor device including a transistorhaving an oxide semiconductor and occupying a small area. Another objectis to provide a semiconductor device including a transistor having anoxide semiconductor and having a stable electrical characteristic.Another object is to provide a semiconductor device including an oxidesemiconductor and having high reliability. Another object is to providea novel semiconductor device. Another object is to provide a noveldisplay device.

Note that the description of the above objects do not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding a transistor and a capacitor. The transistor includes an oxidesemiconductor film, a gate insulating film over the oxide semiconductorfilm, a gate electrode over the gate insulating film, a secondinsulating film over the gate electrode, a third insulating film overthe second insulating film, a source electrode over the third insulatingfilm, and a drain electrode over the third insulating film. The sourceelectrode is electrically connected to the oxide semiconductor film. Thedrain electrode is electrically connected to the oxide semiconductorfilm. The capacitor includes a first conductive film, a secondconductive film, and the second insulating film. The first conductivefilm and the gate electrode are provided over the same surface, thesecond conductive film, the source electrode, and the drain electrodeare provided over the same surface, and the second insulating film isprovided between the first conductive film and the second conductivefilm. More details are described below.

One embodiment of the present invention is a semiconductor deviceincluding a transistor and a capacitor. The transistor includes an oxidesemiconductor film over a first insulating film, a gate insulating filmover the oxide semiconductor film, a gate electrode over the gateinsulating film, a second insulating film over the gate electrode, athird insulating film over the second insulating film, a sourceelectrode over the third insulating film, and a drain electrode over thethird insulating film; the first insulating film includes oxygen; thesecond insulating film includes nitrogen; the source electrode iselectrically connected to the oxide semiconductor film; and the drainelectrode is electrically connected to the oxide semiconductor film. Thecapacitor includes a first conductive film, a second conductive film,and the second insulating film; the first conductive film and the gateelectrode are provided over the same surface; the second conductivefilm, the source electrode, and the drain electrode are provided overthe same surface; and the second insulating film is provided between thefirst conductive film and the second conductive film.

Another embodiment of the present invention is a semiconductor deviceincluding a transistor and a capacitor. The transistor includes a firstgate electrode over a first insulating film, a first gate insulatingfilm over the first gate electrode, an oxide semiconductor film over thefirst gate insulating film, a second gate insulating film over the oxidesemiconductor film, a second gate electrode over the second gateinsulating film, a second insulating film over the second gateelectrode, a third insulating film over the second insulating film, asource electrode over the third insulating film, and a drain electrodeover the third insulating film; the first gate insulating film includesoxygen; the second insulating film includes nitrogen; the sourceelectrode is electrically connected to the oxide semiconductor film; andthe drain electrode is electrically connected to the oxide semiconductorfilm. The capacitor includes a first conductive film, a secondconductive film, and the second insulating film; the first conductivefilm and the second gate electrode are provided over the same surface;the second conductive film, the source electrode, and the drainelectrode are provided over the same surface; and the second insulatingfilm is provided between the first conductive film and the secondconductive film.

In the above embodiment, it is preferable that the oxide semiconductorfilm include a first region and a second region, the first region have aregion overlapping with the gate electrode, the second region have aregion not overlapping with the gate electrode, the first region have aportion in which a concentration of an impurity element is a firstconcentration, the second region have a portion in which a concentrationof the impurity element is a second concentration, and the firstconcentration be different from the second concentration. In the aboveembodiment, it is preferable that the oxide semiconductor film include afirst region and a second region, the first region have a regionoverlapping with the second gate electrode, the second region have aregion not overlapping with the second gate electrode, the first regionhave a portion in which a concentration of an impurity element is afirst concentration, the second region have a portion in which aconcentration of the impurity element is a second concentration, and thefirst concentration be different from the second concentration.

In any of the above embodiments, it is preferable that the impurityelement include one or more of hydrogen, boron, carbon, nitrogen,fluorine, aluminum, silicon, phosphorus, chlorine, and a rare gaselement. In any of the above embodiments, it is preferable that theimpurity element include argon and hydrogen.

In any of the above embodiments, it is preferable that the second regionhave a region in contact with the second insulating film. In any of theabove embodiments, it is preferable that the second region have a regionwith a higher concentration of the impurity element than the firstregion. In any of the above embodiments, it is preferable that the firstregion have a region with higher crystallinity than the second region.

In any of the above embodiments, it is preferable that the oxidesemiconductor film contain oxygen, In, Zn, and M (M is Ti, Ga, Y, Zr,La, Ce, Nd, or Hf). In any of the above embodiments, it is preferablethat the oxide semiconductor film include a crystal part having c-axisalignment and a portion in which the c-axis is parallel to a normalvector of a surface where the oxide semiconductor film is formed.

Another embodiment of the present invention is a display deviceincluding the semiconductor device according to any one of the aboveembodiments and a display element. Another embodiment of the presentinvention is a display module including the display device and a touchsensor. Another embodiment of the present invention is an electronicdevice including the semiconductor device according to any one of theabove embodiments, the display device, or the display module, and anoperation key or a battery.

According to one embodiment of the present invention, a novelsemiconductor device including a transistor having an oxidesemiconductor can be provided. In particular, a semiconductor deviceincluding a planar type transistor having an oxide semiconductor can beprovided. Alternatively, a semiconductor device including a planartransistor having an oxide semiconductor and a capacitor connected tothe transistor can be provided, a semiconductor device including atransistor having an oxide semiconductor and having high on-statecurrent can be provided, a semiconductor device including a transistorhaving an oxide semiconductor and having low off-state current can beprovided, a semiconductor device including a transistor having an oxidesemiconductor and occupying a small area can be provided, asemiconductor device including a transistor having an oxidesemiconductor and having a stable electrical characteristic can beprovided, a semiconductor device including an oxide semiconductor andhaving high reliability can be provided, a novel semiconductor devicecan be provided, or a novel display device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are top views and cross-sectional views which illustrateone embodiment of a semiconductor device.

FIG. 2 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 3A to 3D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 4A and 4B are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 5A to 5D are top views and cross-sectional views which illustrateone embodiment of a semiconductor device.

FIG. 6 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIGS. 7A to 7D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 8A to 8D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 9A to 9D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIG. 10 is a cross-sectional view illustrating one embodiment of asemiconductor device.

FIG. 11A is a cross-sectional view illustrating one embodiment of asemiconductor device and FIGS. 11B and 11C illustrate one embodiment ofa band structure.

FIGS. 12A to 12H are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 13A to 13F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 14A to 14F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 15A to 15F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 16A to 16F are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 17A to 17C are cross-sectional TEM images and a local Fouriertransform image of an oxide semiconductor.

FIGS. 18A and 18B show nanobeam electron diffraction patterns of oxidesemiconductor films, and FIGS. 18C and 18D illustrate an example of atransmission electron diffraction measurement apparatus.

FIG. 19A shows an example of structural analysis by transmissionelectron diffraction measurement and FIGS. 19B and 19C show plan-viewTEM images.

FIG. 20 shows a calculation model.

FIGS. 21A and 21B show an initial state and a final state, respectively.

FIG. 22 shows an activation barrier.

FIGS. 23A and 23B illustrate an initial state and a final state,respectively.

FIG. 24 shows an activation barrier.

FIG. 25 shows the transition levels of V_(o)H.

FIG. 26 is a top view illustrating one embodiment of a display device.

FIG. 27 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 28 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 29A and 29B illustrate a structure of a pixel portion of alight-emitting device.

FIGS. 30A to 30D are cross-sectional views of a semiconductor device.

FIGS. 31A to 31C are a top view and circuit diagrams of a displaydevice.

FIGS. 32A and 32B are a circuit diagram and a timing chart of a displaydevice.

FIGS. 33A and 33B are a circuit diagram and a timing chart of a displaydevice.

FIGS. 34A and 34B are a circuit diagram and a timing chart of a displaydevice.

FIGS. 35A and 35B are a circuit diagram and a timing chart of a displaydevice.

FIG. 36 illustrates a display module.

FIGS. 37A to 37H illustrate electronic devices.

FIGS. 38A and 38B are cross-sectional TEM images in an example.

FIG. 39 shows temperature dependence of resistivity.

FIGS. 40A to 40C schematically illustrates a CAAC-OS deposition model,and FIGS. 40B and 40C are cross-sectional views of pellets and aCAAC-OS.

FIG. 41 is a schematic diagram illustrating an nc-OS deposition modeland a pellet.

FIG. 42 illustrates a pellet.

FIG. 43 illustrates force applied to a pellet on a formation surface.

FIGS. 44A and 44B illustrate movement of a pellet on a formationsurface.

FIGS. 45A and 45B illustrate an InGaZnO₄ crystal.

FIGS. 46A and 46B show a structure and the like of InGaZnO₄ beforecollision of an atom.

FIGS. 47A and 47B show a structure and the like of InGaZnO₄ aftercollision of an atom.

FIGS. 48A and 48B show trajectories of atoms after collision of atoms.

FIGS. 49A and 49B are cross-sectional HAADF-STEM images of a CAAC-OS anda target.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Further, the positional relation between components is changedas appropriate in accordance with a direction in which each component isdescribed. Thus, there is no limitation on terms used in thisspecification, and description can be made appropriately depending onthe situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain region, the channel region, and the source region. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Embodiment 1

In this embodiment, a semiconductor device in which a transistor and acapacitor are provided over the same substrate and a method formanufacturing the semiconductor device are described with reference toFIGS. 1A to 1D, FIG. 2, FIGS. 3A to 3D, FIGS. 4A and 4B, FIGS. 5A to 5D,FIG. 6, FIGS. 7A to 7D, FIGS. 8A to 8D, FIGS. 9A to 9D, FIG. 10, FIGS.11A to 11C, FIGS. 12A to 12H, FIGS. 13A to 13F, FIGS. 14A to 14F, FIGS.15A to 15F, and FIGS. 16A to 16F.

<Structure 1 of Semiconductor Device>

FIGS. 1A to 1D illustrate an example of a semiconductor device in whicha transistor and a capacitor are provided over the same substrate. Notethat the transistor has a top-gate structure.

FIG. 1A is a top view of a transistor 100 included in the semiconductordevice. FIG. 1B is a top view of a capacitor 150 included in thesemiconductor device. FIG. 1C is a cross-sectional view along thedashed-dotted line X1-X2 in FIG. 1A. FIG. 1D is a cross-sectional viewalong the dashed-dotted line X3-X4 in FIG. 1B. Note that in FIGS. 1A and1B, a substrate 102, an insulating film 104, an insulating film 108, aninsulating film 118, an insulating film 120, and the like are notillustrated for simplicity. In a manner similar to that of FIGS. 1A and1B, some components are not illustrated in some cases in top views oftransistors and capacitors described below. Furthermore, the directionof the dashed-dotted line X1-X2 may be called a channel lengthdirection, and the direction of the dashed-dotted line Y1-Y2 may becalled a channel width direction.

The transistor 100 illustrated in FIGS. 1A and 1C includes theinsulating film 108 formed over the substrate 102, an oxidesemiconductor film 110 over the insulating film 108, an insulating film112 over the oxide semiconductor film 110, a conductive film 114overlapping with the oxide semiconductor film 110 with the insulatingfilm 112 provided therebetween, the insulating film 118 covering theoxide semiconductor film 110, the insulating film 112, and theconductive film 114, the insulating film 120 over the insulating film118, a conductive film 122 connected to the oxide semiconductor film 110through an opening portion 140 a provided in the insulating film 118 andthe insulating film 120, and a conductive film 124 connected to theoxide semiconductor film 110 through an opening portion 140 b providedin the insulating film 118 and the insulating film 120. Note that aninsulating film 128 covering the insulating film 120, the conductivefilm 122, and the conductive film 124 may be provided over thetransistor 100.

Note that in FIG. 1C, the insulating film 108 has a stacked-layerstructure of an insulating film 108 a and an insulating film 108 b overthe insulating film 108 a. The conductive film 114 has a stacked-layerstructure of a conductive film 114 a and a conductive film 114 b overthe conductive film 114 a. The conductive film 122 has a stacked-layerstructure of a conductive film 122 a and a conductive film 122 b overthe conductive film 122 a. The conductive film 124 has a stacked-layerstructure of a conductive film 124 a and a conductive film 124 b overthe conductive film 124 a.

In the transistor 100, the conductive film 114 functions as a gateelectrode (also referred to as a top-gate electrode), the conductivefilm 122 functions as one of a source electrode and a drain electrode,and the conductive film 124 functions as the other of the sourceelectrode and the drain electrode. Furthermore, in the transistor 100,the insulating film 108 functions as a base film of the oxidesemiconductor film 110 and the insulating film 112 functions as a gateinsulating film.

The capacitor 150 illustrated in FIGS. 1B and 1D includes the insulatingfilm 108 formed over the substrate 102, the insulating film 112 over theinsulating film 108, a conductive film 116 over the insulating film 112,the insulating film 118 covering the insulating film 108, the insulatingfilm 112, and the conductive film 116, the insulating film 120 over theinsulating film 118, and a conductive film 126 overlapping with theconductive film 116 with the insulating film 118 provided therebetweenin an opening portion 140 c provided in the insulating film 120. Notethat the insulating film 128 covering the insulating film 120 and theconductive film 126 may be provided over the capacitor 150.

Note that in FIG. 1D, the insulating film 108 has a stacked-layerstructure of the insulating film 108 a and the insulating film 108 bover the insulating film 108 a. The conductive film 116 has astacked-layer structure of a conductive film 116 a and a conductive film116 b over the conductive film 116 a. The conductive film 126 has astacked-layer structure of a conductive film 126 a and a conductive film126 b over the conductive film 126 a.

Furthermore, the capacitor 150 has a structure in which a dielectric isprovided between a pair of electrodes. In more detail, one of the pairof electrodes is the conductive film 116, the other of the pair ofelectrodes is the conductive film 126, and the insulating film 118between the conductive film 116 and the conductive film 126 functions asthe dielectric.

Note that the conductive film 114 functioning as the gate electrode ofthe transistor 100 and the conductive film 116 functioning as the one ofthe pair of electrodes of the capacitor 150 are formed in the same step,and the conductive films 114 and 116 are at least partly formed over thesame surface. Furthermore, the conductive film 122 and the conductivefilm 124 that function as the source electrode and the drain electrodeof the transistor 100 and the conductive film 126 functioning as theother of the pair of electrodes of the capacitor 150 are formed in thesame step, and the conductive films 122, 124, and 126 are at leastpartly formed over the same surface.

As described above, by forming the conductive films that function as theelectrodes of the transistor 100 and the capacitor 150 in the same step,a manufacturing cost can be reduced.

Furthermore, in the capacitor 150, the insulating film 120 has theopening portion 140 c. Therefore, in an insulating film in which theinsulating film 118 and the insulating film 120 are stacked, only theinsulating film 118 is made to function as the dielectric. The capacitor150 having such a structure can have a high capacitance value, andaccordingly, a display device can have a high capacitance value.

FIG. 2 is a cross-sectional view of the transistor 100 illustrated inFIG. 1A in the dashed-dotted line Y1-Y2 direction (the channel widthdirection).

As illustrated in FIG. 2, an end portion of the conductive film 114 a ispositioned on the outer side than an end portion of the conductive film114 b in the channel width direction. Furthermore, an end portion of theinsulating film 112 is positioned on the outer side than the end portionof the conductive film 114 a. Furthermore, the insulating film 108 b hasa depressed portion in a region that does not overlap with theinsulating film 112. By using such a structure, the coverage with theinsulating films 118, 120, and 128 can be increased.

Next, the oxide semiconductor film 110 included in the transistor 100 isdescribed in detail below.

An element which forms an oxygen vacancy is contained in a region thatdoes not overlap with the conductive film 114 in the oxide semiconductorfilm 110 of the transistor 100. Hereinafter, elements which form oxygenvacancies are described as impurity elements. Typical examples ofimpurity elements are hydrogen, boron, carbon, nitrogen, fluorine,aluminum, silicon, phosphorus, chlorine, and rare gas elements. Typicalexamples of rare gas elements are helium, neon, argon, krypton, andxenon.

When the impurity element is added to the oxide semiconductor film, abond between a metal element and oxygen in the oxide semiconductor filmis cut, whereby an oxygen vacancy is formed. When the impurity elementis added to the oxide semiconductor film, oxygen bonded to a metalelement in the oxide semiconductor film is bonded to the impurityelement, whereby oxygen is detached from the metal element andaccordingly an oxygen vacancy is formed. As a result, the oxidesemiconductor film has a higher carrier density and thus theconductivity thereof becomes higher.

When hydrogen is added to an oxide semiconductor in which an oxygenvacancy is generated by addition of the impurity element, hydrogenenters an oxygen vacant site and forms a donor level in the vicinity ofthe conduction band. As a result, the conductivity of the oxidesemiconductor is increased, so that the oxide semiconductor becomes aconductor. An oxide semiconductor having become a conductor can bereferred to as an oxide conductor. Oxide semiconductors generally have avisible light transmitting property because of their large energy gap.An oxide conductor is an oxide semiconductor having a donor level in thevicinity of the conduction band. Therefore, the influence of absorptiondue to the donor level is small, and an oxide conductor has a visiblelight transmitting property comparable to that of an oxidesemiconductor.

Here, the temperature dependence of resistivity of a film formed with anoxide conductor (hereinafter referred to as an oxide conductor film) isdescribed with reference to FIG. 39.

Here, a sample including an oxide conductor film was formed. As theoxide conductor film, an oxide conductor film (OC_SiN_(x)) formed bymaking the oxide semiconductor film in contact with a silicon nitridefilm, an oxide conductor film (OC_Ar dope+SiN_(x)) formed by making theoxide semiconductor film in contact with a silicon nitride film afteraddition of argon to the oxide semiconductor film with a dopingapparatus, or an oxide conductor film (OC_Ar plasma+SiN_(x)) formed bymaking the oxide semiconductor film in contact with a silicon nitridefilm after exposure of the oxide semiconductor film to argon plasma witha plasma treatment apparatus was formed. The silicon nitride filmcontains hydrogen.

A method for forming a sample including the oxide conductor film(OC_SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride film wasformed over a glass substrate by a plasma CVD method and then exposed tooxygen plasma so that an oxygen ion was added to the silicon oxynitridefilm, whereby a silicon oxynitride film that releases oxygen by beingheated was formed. Next, a 100-nm-thick In—Ga—Zn oxide film was formedover the silicon oxynitride film that releases oxygen by being heated bya sputtering method using a sputtering target in which the atomic ratioof In to Ga and Zn was 1:1:1.2, and heat treatment was performed at 450°C. in a nitrogen atmosphere and then heat treatment was performed at450° C. in a mixed atmosphere of nitrogen and oxygen. Next, a100-nm-thick silicon nitride film was formed by a plasma CVD method.Then, the film was subjected to heat treatment in a mixed gas ofnitrogen and oxygen at 350° C.

A method for forming a sample including the oxide conductor film (OC_Ardope+SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride film wasformed over a glass substrate by a plasma CVD method and then exposed tooxygen plasma so that an oxygen ion was added to the silicon oxynitridefilm, whereby a silicon oxynitride film that releases oxygen by beingheated was formed. Next, a 100-nm-thick In—Ga—Zn oxide film was formedover the silicon oxynitride film that releases oxygen by being heated bya sputtering method using a sputtering target in which the atomic ratioof In to Ga and Zn was 1:1:1.2, and heat treatment was performed at 450°C. in a nitrogen atmosphere and then heat treatment was performed at450° C. in a mixed atmosphere of nitrogen and oxygen. Next, with adoping apparatus, argon with a dose of 5×10¹⁴ ions/cm² was added to theIn—Ga—Zn oxide film at an accelerating voltage of 10 kV, and oxygenvacancies were formed in the In—Ga—Zn oxide film. Next, a 100-nm-thicksilicon nitride film was formed by a plasma CVD method. Then, the filmwas subjected to heat treatment in a mixed gas of nitrogen and oxygen at350° C.

A method for forming a sample including the oxide conductor film (OC_Arplasma+SiN_(x)) is as follows. A 400-nm-thick silicon oxynitride filmwas formed over a glass substrate by a plasma CVD method and thenexposed to oxygen plasma, whereby a silicon oxynitride film thatreleases oxygen by being heated was formed. Next, a 100-nm-thickIn—Ga—Zn oxide film was formed over the silicon oxynitride film thatreleases oxygen by being heated by a sputtering method using asputtering target in which the atomic ratio of In to Ga and Zn was1:1:1.2, and heat treatment was performed at 450° C. in a nitrogenatmosphere and then heat treatment was performed at 450° C. in a mixedatmosphere of nitrogen and oxygen. Then, in a plasma treatmentapparatus, argon plasma was generated, accelerated argon ions were madeto collide with the In—Ga—Zn oxide film, and oxygen vacancies wereformed in the In—Ga—Zn oxide film. Next, a 100-nm-thick silicon nitridefilm was formed by a plasma CVD method. Then, the film was subjected toheat treatment in a mixed gas of nitrogen and oxygen at 350° C.

Next, FIG. 39 shows the measured resistivity of the samples. Here, theresistivity was measured by the Van der Pauw method using fourterminals. In FIG. 39, the horizontal axis represents measurementtemperature, and the vertical axis represents resistivity. Measurementresults of the oxide conductor film (OC_SiN_(x)) are plotted as squares,measurement results of the oxide conductor film (OC_Ar dope+SiN_(x)) areplotted as circles, and measurement results of the oxide conductor film(OC_Ar plasma+SiN_(x)) are plotted as triangles.

Note that although not shown, the oxide semiconductor film which is notin contact with the silicon nitride film had high resistivity, which wasdifficult to measure. Therefore, it is found that the oxide conductorfilm has lower resistivity than the oxide semiconductor film.

According to FIG. 39, in the case where the oxide conductor film (OC_Ardope+SiN_(x)) and the oxide conductor film (OC_Ar plasma+SiN_(x))contain an oxygen vacancy and hydrogen, variation in resistivity issmall. Typically, the variation in resistivity at temperatures from 80 Kto 290 K is lower than ±20%. Alternatively, the variation in resistivityat temperatures from 150 K to 250 K is lower than +10%. In other words,the oxide conductor is a degenerate semiconductor and it is suggestedthat the conduction band edge agrees with or substantially agrees withthe Fermi level. Thus, when the oxide conductor film is used as a sourceregion and a drain region of a transistor, an ohmic contact is made at aportion where the oxide conductor film is in contact with a conductivefilm functioning as a source electrode and a drain electrode, and thecontact resistance of the oxide conductor film and the conductive filmfunctioning as a source electrode and a drain electrode can be reduced.Furthermore, the oxide conductor has low temperature dependence ofresistivity; thus, a fluctuation of contact resistance of the oxideconductor film and a conductive film functioning as a source electrodeand a drain electrode is small, and a highly reliable transistor can beobtained.

FIGS. 3A to 3D and FIGS. 4A and 4B are enlarged views of the vicinity ofthe oxide semiconductor film 110. Note that in FIGS. 3A to 3D and FIGS.4A and 4B, some components are not illustrated in order to avoidcomplexity.

A region in which the carrier density of the oxide semiconductor film isincreased and the conductivity thereof is increased (hereinafter such aregion is referred to as a low-resistance region) is formed in a crosssection of the oxide semiconductor film 110 in the channel lengthdirection. Furthermore, low-resistance regions formed in the oxidesemiconductor film 110 can have a plurality of structures as illustratedin FIGS. 3A to 3D and FIGS. 4A and 4B. Note that in FIGS. 3A to 3D andFIGS. 4A and 4B, a channel length L corresponds to a length of a regionbetween a pair of low-resistance regions.

As illustrated in FIG. 3A, the oxide semiconductor film 110 includes achannel region 110 a formed in a region overlapping with the conductivefilm 114 and low-resistance regions 110 b and 110 c between which thechannel region 110 a is provided and which contain the impurityelements. Note that as illustrated in FIG. 3A, in the cross-sectionalshape in the channel length direction, the boundaries between thechannel region 110 a and the low-resistance regions 110 b and 110 ccoincide with or substantially coincide with bottom end portions of theconductive film 114 a, with the insulating film 112 provided between theconductive film 114 a and the boundaries. That is, in a top surfaceshape, the boundaries between the channel region 110 a and thelow-resistance regions 110 b and 110 c coincide with or substantiallycoincide with the bottom end portions of the conductive film 114 a.

Note that as illustrated in FIG. 3A, in the cross-sectional shape in thechannel length direction, the end portion of the conductive film 114 amay be positioned on the outer side than the end portion of theconductive film 114 b and the conductive film 114 b may have a taperedshape. That is, an angle θ1 formed between a surface where theconductive film 114 a and the conductive film 114 b are in contact witheach other and a side surface of the conductive film 114 b may be lessthan 90°, greater than or equal to 10° and less than or equal to 85°,greater than or equal to 15° and less than or equal to 85°, greater thanor equal to 30° and less than or equal to 85°, greater than or equal to45° and less than or equal to 85°, or greater than or equal to 60° andless than or equal to 85°. When the angle θ1 is less than 90°, greaterthan or equal to 10° and less than or equal to 85°, greater than orequal to 15° and less than or equal to 85°, greater than or equal to 30°and less than or equal to 85°, greater than or equal to 45° and lessthan or equal to 85°, or greater than or equal to 60° and less than orequal to 85°, the coverage of the side surfaces of the insulating film114 b with the insulating film 118 can be increased.

As illustrated in FIG. 3A, in the cross-sectional shape in the channellength direction, the end portion of the insulating film 112 may bepositioned on the outer side than the end portions of the conductivefilm 114 a and the conductive film 114 b. The end portion of theinsulating film 112 may be partly arc-shaped. Alternatively, theinsulating film 112 may have a tapered shape. That is, an angle θ2formed between a surface where the oxide semiconductor film 110 and theinsulating film 112 are in contact with each other and a side surface ofthe insulating film 112 may be less than 90°, preferably greater than orequal to 30° and less than 90°.

Alternatively, as illustrated in FIG. 3B, in a cross-sectional shape inthe channel length direction, the low-resistance regions 110 b and 110 ceach have a region overlapping with the conductive film 114 with theinsulating film 112 provided therebetween. The regions function as anoverlap region. The overlap region in the channel length direction isreferred to as L_(ov). L_(ov) is smaller than 20%, smaller than 10%,smaller than 5%, or smaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 3C, in a cross-sectional shape inthe channel length direction, the channel region 110 a has a region thatdoes not overlap with the bottom end portion of the conductive film 114a. The region functions as an offset region. The length of the offsetregion in the channel length direction is referred to as L_(off). Notethat when a plurality of offset regions is provided, L_(off) indicatesthe length of one offset region. L_(off) is included in the channellength L. Note that L_(off) is smaller than 20%, smaller than 10%,smaller than 5%, or smaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 3D, in a cross-sectional shape inthe channel length direction, the oxide semiconductor film 110 includesa low-resistance region 110 d between the channel region 10 a and thelow-resistance region 110 b, and a low-resistance region 110 e betweenthe channel region 110 a and the low-resistance region 110 c. Thelow-resistance regions 110 d and 110 e have lower impurity elementconcentrations and higher resistivity than the low-resistance regions110 b and 110 c. Here, the low-resistance regions 110 d and 110 eoverlap with the insulating film 112, but they may overlap with theinsulating film 112 and the conductive film 114.

Alternatively, as illustrated in FIG. 4A, in a cross-sectional shape inthe channel length direction, the oxide semiconductor film 110 includesregions 110 f and 110 g in regions overlapping with the conductive films122 and 124. The impurity element is not necessarily added to theregions 110 f and 110 g. In this case, the oxide semiconductor film 110includes regions containing the impurity elements, i.e., thelow-resistance regions 110 b and 110 c. The low-resistance region (110 bor 110 c) is provided between the channel region 110 a and the region(110 f or 110 g) in contact with the conductive film (122 or 124). Theregions 110 f and 110 g have conductivity when voltage is applied to theconductive films 122 and 124; thus, the regions 110 f and 110 g functionas a source region and a drain region.

Note that the structure illustrated in FIG. 4A is formed as follows:after the conductive films 122 and 124 are formed, the impurity elementis added to the oxide semiconductor film 110 through the insulating film120 and the insulating film 118 using the conductive films 114, 122, and124 as masks.

Alternatively, as illustrated in FIG. 4B, in a cross-sectional shape inthe channel length direction, the low-resistance regions 110 b, 110 c,110 d, 110 e, 110 h, and 110 i between which the channel region 110 a isprovided may be provided.

Specifically, the oxide semiconductor film 110 illustrated in FIG. 4Bincludes the channel region 110 a, the low-resistance regions 110 h and110 i between which the channel region 110 a is provided, thelow-resistance regions 110 d and 110 e between which the low-resistanceregions 110 h and 110 i are provided, and the low-resistance regions 110b and 110 c between which the low-resistance regions 100 d and 110 e areprovided. The low-resistance regions 110 h and 110 i are formed byadding the impurity element through regions of the conductive film 114 aand the insulating film 112 not overlapping with the conductive film 114b. The low-resistance regions 110 d and 110 e are formed by adding theimpurity element through regions of the insulating film 112 notoverlapping with the conductive film 114 a and the conductive film 114b. The low-resistance regions 110 b and 110 c are formed by directlyadding the impurity element. Therefore, the low-resistance regions 110 hand 110 i have lower impurity concentrations and higher resistivity thanthe low-resistance regions 110 d and 110 e and the low-resistanceregions 110 b and 110 c. Furthermore, the low-resistance regions 110 dand 110 e have lower impurity concentrations and higher resistivity thanthe low-resistance regions 110 b and 110 c.

Note that in FIG. 4B, the channel region 110 a overlaps with theconductive film 114 b. Furthermore, the low-resistance regions 110 h and110 i overlap with the conductive film 114 a projecting outside theconductive film 114 b. Furthermore, the low-resistance regions 110 d and110 e overlap with the insulating film 112 projecting outside theconductive film 114 a. Furthermore, the low-resistance regions 110 b and110 c project outside the insulating film 112 and overlap with theinsulating film 118.

As illustrated in FIG. 3D and FIG. 4B, the oxide semiconductor film 110includes the low-resistance regions 110 d, 110 e, 110 h, and 110 ihaving lower impurity element concentrations and higher resistivity thanthe low-resistance regions 110 b and 110 c, whereby the electric fieldof the drain region can be relaxed. Thus, change in the thresholdvoltage of the transistor due to the electric field of the drain regioncan be reduced.

The oxide semiconductor films 110 illustrated in FIGS. 3A to 3D andFIGS. 4A and 4B each include a region that does not overlap with theinsulating film 112 and the conductive film 114 and is thinner than aregion of the oxide semiconductor film 110 overlapping with theinsulating film 112 and the conductive film 114. The thin region isthinner than the region of the oxide semiconductor film overlapping withthe insulating film 112 and the conductive film 114; the thickness ofthe thin region Is greater than or equal to 0.1 nm and less than orequal to 5 nm.

Note that the low-resistance regions 110 b and 110 c in the oxidesemiconductor film 110 function as a source region and a drain region.Furthermore, the impurity element is contained in the low-resistanceregions 110 b and 110 c and the low-resistance regions 110 d, 110 e, 110h, and 110 i.

In the case where the impurity element is a rare gas element and theoxide semiconductor film 110 is formed by a sputtering method, thechannel region 110 a and the low-resistance regions 110 b, 110 c, 110 d,110 e, 110 h, and 110 i each contain a rare gas element. Note that theconcentrations of the rare gas elements in the low-resistance regions110 b and 110 c are higher than the concentration of the rare gaselement in the channel region 110 a. Furthermore, the concentrations ofthe rare gas elements in the low-resistance regions 110 b and 110 c arehigher than the concentrations of the rare gas elements in thelow-resistance regions 110 d and 110 e. Furthermore, the concentrationsof the rare gas elements in the low-resistance regions 110 d and 110 eare higher than the concentrations of the rare gas elements in thelow-resistance regions 110 h and 110 i.

The reasons for this are as follows: in the case where the oxidesemiconductor film 110 is formed by a sputtering method, a rare gas isused as a sputtering gas, so that the oxide semiconductor film 110contains the rare gas; and a rare gas is intentionally added to thelow-resistance regions 110 b and 110 c in order to form oxygen vacanciesin the low-resistance regions 110 b and 110 c. Furthermore, thelow-resistance regions 110 d, 110 e, 110 h, and 110 i have differentconcentrations of rare gas elements added to form oxygen vacancies. Thedifference in rare gas concentration is due to a difference in thestructures and thicknesses of films formed over the low-resistanceregions 110 d, 110 e, 110 h, and 110 i. Note that a rare gas elementdifferent from the rare gas element contained in the channel region 110a may be added to the low-resistance regions 110 b, 110 c, 110 d, 110 e,110 h, and 110 i.

In the case where the impurity element is boron, carbon, nitrogen,fluorine, aluminum, silicon, phosphorus, or chlorine, the low-resistanceregions 110 b, 110 c, 110 d, 110 e, 110 h, and 110 i contain theabove-described impurity element. Therefore, the concentrations of theimpurity elements in the low-resistance regions 110 b, 110 c, 110 d, 110e, 110 h, and 110 i are higher than the concentration of the impurityelement in the channel region 110 a. Note that the concentrations of theimpurity elements in the low-resistance regions 110 b, 110 c, 110 d, 110e, 110 h, and 110 i which are measured by secondary ion massspectrometry (SIMS) can be greater than or equal to 5×10¹⁸ atoms/cm³ andless than or equal to 1×10²² atoms/cm³, greater than or equal to 1×10¹⁹atoms/cm³ and less than or equal to 1×10²¹ atoms/cm³, or greater than orequal to 5×10¹⁹ atoms/cm³ and less than or equal to 5×10²⁰ atoms/cm³.

The impurity element concentrations in the low-resistance regions 110 b,110 c, 110 d, 110 e, 110 h, and 110 i are higher than those in thechannel region 110 a in the case where the impurity elements arehydrogen. Note that the concentrations of hydrogen in the low-resistanceregions 110 b, 110 c, 110 d, 110 e, 110 h, and 110 i which are measuredby SIMS can be higher than or equal to 8×10¹⁹ atoms/cm³, higher than orequal to 1×10²⁰ atoms/cm³, or higher than or equal to 5×10²⁰ atoms/cm³.

Since the low-resistance regions 110 b, 110 c, 110 d, 110 e, 110 h, and110 i contain the impurity elements, oxygen vacancies and carrierdensities are increased. As a result, the low-resistance regions 110 b,110 c, 110 d, 110 e, 110 h, and 110 i have higher conductivity.

Note that the impurity element may be a combination of a rare gas andone or more of hydrogen, boron, carbon, nitrogen, fluorine, aluminum,silicon, phosphorus, and chlorine. In that case, in the low-resistanceregions 110 b, 110 c, 110 d, 110 e, 110 h, and 110 i, by interactionbetween oxygen vacancies formed by the rare gas and one or more ofhydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon,phosphorus, and chlorine which is added, the conductivity of thelow-resistance regions 110 b, 110 c, 110 d, 110 e, 110 h, and 110 i isfurther increased in some cases.

Furthermore, when hydrogen is added to an oxide semiconductor in whichan oxygen vacancy is generated by addition of the impurity element,hydrogen enters an oxygen vacant site and forms a donor level in thevicinity of the conduction band. Consequently, an oxide conductor can beformed. Accordingly, the oxide conductor has a light-transmittingproperty. Here, an oxide conductor refers to an oxide semiconductorhaving become a conductor.

The oxide conductor is a degenerate semiconductor and it is suggestedthat the conduction band edge agrees with or substantially agrees withthe Fermi level. For that reason, an ohmic contact is made between theoxide conductor film and the conductive films functioning as a sourceelectrode and a drain electrode; thus, contact resistance between theoxide conductor film and the conductive films functioning as a sourceelectrode and a drain electrode can be reduced.

In the transistor 100 described in this embodiment, the channel region110 a is sandwiched between the low-resistance regions 110 b and 110 cfunctioning as a source region and a drain region. Therefore, theon-state current and field-effect mobility of the transistor 100 arehigh. In addition, in the transistor 100, the impurity element is addedto the oxide semiconductor film 110 using the conductive film 114 as amask. That is, the low-resistance region can be formed in a self-alignedmanner.

Furthermore, in the transistor 100, the conductive film 114 functioningas a gate electrode does not overlap with the conductive films 122 and124 functioning as a source electrode and a drain electrode. Therefore,parasitic capacitance between the conductive film 114 and the conductivefilms 122 and 124 can be reduced. As a result, in the case where alarge-area substrate is used as the substrate 102, signal delay in theconductive film 114 and the conductive films 122 and 124 can be reduced.

Next, details of other elements included in the semiconductor deviceillustrated in FIGS. 1A to 1D are described.

The type of the substrate 102 is not limited to a certain type, and anyof a variety of substrates can be used as the substrate 102. As thesubstrate, a semiconductor substrate (e.g., a single crystal substrateor a silicon substrate), an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a metal substrate, a stainless steelsubstrate, a substrate including stainless steel foil, a tungstensubstrate, a substrate including tungsten foil, a flexible substrate, anattachment film, paper including a fibrous material, a base materialfilm, or the like can be used, for example. As an example of a glasssubstrate, a barium borosilicate glass substrate, an aluminoborosilicateglass substrate, soda lime glass substrate, and the like can be given.Examples of the flexible substrate, the attachment film, and the basematerial film are plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and polyether sulfone (PES), a syntheticresin of acrylic or the like, polyester, polypropylene, polyvinylfluoride, polyvinyl chloride, polyamide, polyimide, aramid, epoxy, aninorganic vapor deposition film, paper, and the like. Specifically, whena transistor and a capacitor are formed using a semiconductor substrate,a single crystal substrate, an SOI substrate, or the like, it ispossible to form a transistor and a capacitor with few variations incharacteristics, size, shape, or the like and with high current supplycapability and a small size. By forming a circuit using such atransistor and a capacitor, power consumption of the circuit can bereduced or the circuit can be highly integrated.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor and the capacitor may be provided directly on theflexible substrate. Alternatively, a separation layer may be providedbetween the substrate 102, and the transistor and the capacitor. Theseparation layer can be used when part or the whole of a semiconductordevice formed over the separation layer is separated from the substrate102 and transferred onto another substrate. In such a case, thetransistor and the capacitor can be transferred to a substrate havinglow heat resistance or a flexible substrate as well. For the aboveseparation layer, a stack including inorganic films, which are atungsten film and a silicon oxide film, or an organic resin film ofpolyimide or the like formed over a substrate can be used, for example.

Examples of the substrate to which the transistor and the capacitor aretransferred include, in addition to the above-described substrates overwhich the transistor and the capacitor can be formed, a paper substrate,a cellophane substrate, an aramid film substrate, a polyimide filmsubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, and the like. By using such asubstrate, a transistor with excellent properties or a transistor withlow power consumption can be formed, a device with high durability canbe formed, heat resistance can be provided, or reduction in weight orthickness can be achieved.

The insulating film 108 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. Theinsulating film 108 can be formed with a single layer or a stack usingan oxide insulating film or a nitride insulating film. Note that anoxide insulating film is preferably used for at least a region of theinsulating film 108 which is in contact with the oxide semiconductorfilm 110, in order to improve characteristics of the interface with theoxide semiconductor film 110. An oxide insulating film that releasesoxygen by being heated is preferably used as the insulating film 108, inwhich case oxygen contained in the insulating film 108 can be moved tothe oxide semiconductor film 110 by heat treatment.

The thickness of the insulating film 108 can be greater than or equal to50 nm, greater than or equal to 100 nm and less than or equal to 3000nm, or greater than or equal to 200 nm and less than or equal to 1000nm. With use of the thick insulating film 108, the amount of oxygenreleased from the insulating film 108 can be increased, and theinterface state density at the interface between the insulating film 108and the oxide semiconductor film 110 and oxygen vacancy included in thechannel region 110 a of the oxide semiconductor film 110 can be reduced.

The insulating film 108 can be formed with a single layer or a stackusing, for example, silicon oxide, silicon oxynitride, silicon nitrideoxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide,and a Ga—Zn oxide. In this embodiment, a silicon nitride film is used asthe insulating film 108 a, and a silicon oxynitride film is used as theinsulating film 108 b.

The oxide semiconductor film 110 is typically formed using a metal oxidesuch as an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M is Ti,Ga, Y, Zr, La, Ce, Nd, or Hf). Note that the oxide semiconductor film110 has a light-transmitting property.

Note that in the case where the oxide semiconductor film 110 is anIn-M-Zn oxide, when the summation of In and M is assumed to be 100atomic %, the proportions of In and M are as follows: the proportions ofIn and M are preferably set to be greater than or equal to 25 atomic %and less than 75 atomic %, respectively, or greater than or equal to 34atomic % and less than 66 atomic %, respectively.

The energy gap of the oxide semiconductor film 110 is 2 eV or more, 2.5eV or more, or 3 eV or more.

The thickness of the oxide semiconductor film 110 can be greater than orequal to 3 nm and less than or equal to 200 nm, greater than or equal to3 nm and less than or equal to 100 nm, or greater than or equal to 3 nmand less than or equal to 60 nm.

In the case where the oxide semiconductor film 110 is an In-M-Zn oxide,it is preferable that the atomic ratio of metal elements of a sputteringtarget used for forming a film of the In-M-Zn oxide satisfy In ≥M andZn≥M. As the atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5, In:M:Zn=2:1:2.3,In:M:Zn=2:1:3, In:M:Zn=3:1:2, or the like is preferable. Note that theatomic ratios of metal elements in the formed oxide semiconductor film110 vary from the above atomic ratio of metal elements of the sputteringtarget within a range of ±40% as an error.

When silicon or carbon that is one of elements belonging to Group 14 iscontained in the oxide semiconductor film 110, oxygen vacancies areincreased in the oxide semiconductor film 110, and the oxidesemiconductor film 110 becomes an n-type film. Thus, the concentrationof silicon or carbon (the concentration is measured by SIMS) of theoxide semiconductor film 110, in particular, the channel region 110 a,can be lower than or equal to 2×10¹⁸ atoms/cm³, or lower than or equalto 2×10¹⁷ atoms/cm³. As a result, the transistor has positive thresholdvoltage (normally-off characteristics).

Further, the concentration of alkali metal or alkaline earth metal ofthe oxide semiconductor film 110, in particular, the channel region 110a, which is measured by SIMS, can be lower than or equal to 1×10¹⁸atoms/cm³, or lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal andalkaline earth metal might generate carriers when bonded to an oxidesemiconductor, in which case the off-state current of the transistormight be increased. Therefore, it is preferable to reduce theconcentration of alkali metal or alkaline earth metal in the channelregion 110 a. As a result, the transistor has positive threshold voltage(normally-off characteristics).

Furthermore, when nitrogen is contained in the oxide semiconductor film110, in particular, the channel region 110 a, electrons serving ascarriers are generated, carrier density is increased, and the regionbecomes an n-type in some cases. Thus, a transistor including an oxidesemiconductor film which contains nitrogen is likely to have normally-oncharacteristics. For this reason, nitrogen in the oxide semiconductorfilm, in particular, the channel region 110 a, is preferably reduced asmuch as possible. The concentration of nitrogen measured by SIMS can beset to be, for example, less than or equal to 5×10¹⁸ atoms/cm³.

When the impurity element in the oxide semiconductor film 110, inparticular, the channel region 110 a, is reduced, the carrier density ofthe oxide semiconductor film can be lowered. Therefore, in the oxidesemiconductor film 110, in particular, the channel region 110 a, carrierdensity can be less than or equal to 1×10¹⁷/cm³, less than or equal to1×10¹⁵/cm³, less than or equal to 1×10¹³/cm³, less than or equal to1×10¹¹/cm³, or greater than or equal to 1×10⁻⁹/cm³ and less than orequal to 1×10¹⁰/cm³.

Note that an oxide semiconductor film with a low impurity concentrationand a low density of defect states can be used for the oxidesemiconductor film 110, in which case the transistor can have moreexcellent electrical characteristics. Here, the state in which impurityconcentration is low and density of defect states is low (the amount ofoxygen vacancies is small) is referred to as “highly purified intrinsic”or “substantially highly purified intrinsic”. A highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorhas few carrier generation sources, and thus has a low carrier densityin some cases. Thus, a transistor including the oxide semiconductor filmin which a channel region is formed is likely to have positive thresholdvoltage (normally-off characteristics). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has few carrier traps insome cases. Further, a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has an extremely lowoff-state current; the off-state current can be less than or equal tothe measurement limit of a semiconductor parameter analyzer, i.e., lessthan or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between asource electrode and a drain electrode of from 1 V to 10 V. Thus, thetransistor whose channel region is formed in the oxide semiconductorfilm has a small variation in electrical characteristics and highreliability in some cases.

The oxide semiconductor film 110 may have a non-single-crystalstructure, for example. The non-single-crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure described later, or an amorphous structure, for example. Amongthe non-single-crystal structures, the amorphous structure has thehighest density of defect levels, whereas CAAC-OS has the lowest densityof defect levels.

Note that the oxide semiconductor film 110 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Further, the mixed film has astacked-layer structure including, for example, two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure in some cases.

Note that in the oxide semiconductor film 110, the crystallinity of thechannel region 110 a is different from the crystallinity of each of thelow-resistance regions 110 b, 110 c, 110 d, 110 e, 110 h, and 110 i insome cases. Specifically, in the oxide semiconductor film 110, thecrystallinity. of the channel region 110 a is higher than thecrystallinity of each of the low-resistance regions 110 b, 110, 110 d,110 e, 110 h, and 110 i. This is because, when the impurity element isadded to the low-resistance regions 110 b, 110 c, 110 d, 110 e, 110 h,and 110 i, the low-resistance regions 110 b, 110 c, 110 d, 110 e, 110 h,and 110 i are damaged and thus have lower crystallinity.

The insulating film 112 can be formed with a single layer or a stackusing an oxide insulating film or a nitride insulating film. Note thatan oxide insulating film is preferably used for at least a region of theinsulating film 112 which is in contact with the oxide semiconductorfilm 110, in order to improve characteristics of the interface with theoxide semiconductor film 110. The insulating film 112 can be formed witha single layer or a stack using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide,hafnium oxide, gallium oxide, or a Ga—Zn oxide.

Furthermore, it is possible to prevent outward diffusion of oxygen fromthe oxide semiconductor film 110 and entry of hydrogen, water, or thelike into the oxide semiconductor film 110 from the outside by providingan insulating film having a blocking effect against oxygen, hydrogen,water, and the like as the insulating film 112. As the insulating filmwhich has a blocking effect against oxygen, hydrogen, water, and thelike, an aluminum oxide film, an aluminum oxynitride film, a galliumoxide film, a gallium oxynitride film, an yttrium oxide film, an yttriumoxynitride film, a hafnium oxide film, a hafnium oxynitride film, or thelike can be used.

The insulating film 112 may be formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

An oxide insulating film that releases oxygen by being heated ispreferably used as the insulating film 112, in which case oxygencontained in the insulating film 112 can be moved to the oxidesemiconductor film 110 by heat treatment.

The thickness of the insulating film 112 can be greater than or equal to5 nm and less than or equal to 400 nm, greater than or equal to 5 nm andless than or equal to 300 nm, or greater than or equal to 10 nm and lessthan or equal to 250 nm.

The conductive film 114, the conductive film 116, the conductive film122, the conductive film 124, and the conductive film 126 can be formedby a sputtering method, a vacuum evaporation method, a pulsed laserdeposition (PLD) method, a thermal CVD method, or the like. Each of theconductive film 114, the conductive film 116, the conductive film 122,the conductive film 124, and the conductive film 126 can be formedusing, for example, a metal element selected from aluminum, chromium,copper, tantalum, titanium, molybdenum, nickel, iron, cobalt, andtungsten; an alloy containing any of these metal elements as acomponent; an alloy containing these metal elements in combination; orthe like. Furthermore, one or more metal elements selected frommanganese and zirconium may be used. Furthermore, the conductive film114, the conductive film 116, the conductive film 122, the conductivefilm 124, and the conductive film 126 may have a single-layer structureor a stacked-layer structure of two or more layers. For example, any ofthe following can be used: a single-layer structure of an aluminum filmcontaining silicon; a single-layer structure of a copper film containingmanganese; a two-layer structure in which a titanium film is stackedover an aluminum film; a two-layer structure in which a titanium film isstacked over a titanium nitride film; a two-layer structure in which atungsten film is stacked over a titanium nitride film; a two-layerstructure in which a tungsten film is stacked over a tantalum nitridefilm or a tungsten nitride film; a two-layer structure in which a copperfilm is stacked over a copper film containing manganese; a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in this order, a three-layer structure in which acopper film containing manganese, a copper film, and a copper filmcontaining manganese are stacked in this order; and the like.Alternatively, an alloy film or a nitride film which contains aluminumand one or more selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

Note that the conductive film 114 and the conductive film 116 includethe same material and the same stacked-layer structure because they areformed at the same time. Furthermore, the conductive film 122, theconductive film 124, and the conductive film 126 include the samematerial and the same stacked-layer structure because they are formed atthe same time.

The conductive film 114, the conductive film 116, the conductive film122, the conductive film 124, and the conductive film 126 can also beformed using a light-transmitting conductive material such as indium tinoxide (hereinafter also referred to as ITO), indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide containing silicon oxide.It is also possible to have a stacked-layer structure formed using theabove light-transmitting conductive material and the above metalelement.

The thicknesses of the conductive film 114, the conductive film 116, theconductive film 122, the conductive film 124, and the conductive film126 each can be greater than or equal to 30 nm and less than or equal to500 nm, or greater than or equal to 100 nm and less than or equal to 400nm.

A nitride insulating film is used for the insulating film 118. Thenitride insulating film can be formed using silicon nitride, siliconnitride oxide, aluminum nitride, aluminum nitride oxide, or the like.The hydrogen concentration of the insulating film 118 is preferablyhigher than or equal to 1×10²² atoms/cm³. Furthermore, the insulatingfilm 118 is in contact with the low-resistance region of the oxidesemiconductor film 110. Thus, hydrogen contained in the insulating film118 is diffused to the low-resistance region of the oxide semiconductorfilm 110, whereby the hydrogen concentration of the low-resistanceregion is higher than that of the channel region in the oxidesemiconductor film 110.

The insulating film 120 can be formed with a single layer or a stackusing an oxide insulating film or a nitride insulating film. Theinsulating film 120 can be formed with a single layer or a stack using,for example, silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and aGa—Zn oxide.

The insulating film 128 is preferably a film functioning as a barrierfilm against hydrogen, water, and the like from the outside. Theinsulating film 128 can be formed with a single layer or a stack using,for example, silicon nitride, silicon nitride oxide, aluminum oxide, orthe like.

The thicknesses of the insulating film 118, the insulating film 120, andthe insulating film 128 each can be greater than or equal to 30 nm andless than or equal to 500 nm, or greater than or equal to 100 nm andless than or equal to 400 nm.

<Structure 2 of Semiconductor Device>

Another structure of the semiconductor device illustrated in FIGS. 1A to1D is described with reference to FIGS. 5A to 5D and FIG. 6.

FIG. SA is a top view of a transistor 100A included in a semiconductordevice. FIG. 5B is a top view of a capacitor 150A included in thesemiconductor device. FIG. 5C is a cross-sectional view along thedashed-dotted line X1-X2 in FIG. 5A. FIG. 5D is a cross-sectional viewalong the dashed-dotted line X3-X4 in FIG. 5B.

The transistor 100A illustrated in FIGS. 5A and 5C includes theinsulating film 104 formed over the substrate 102, a conductive film 106over the insulating film 104, the insulating film 108 over theinsulating film 104 and the conductive film 106, the oxide semiconductorfilm 110 overlapping with the conductive film 106 with the insulatingfilm 108 provided therebetween, the insulating film 112 over the oxidesemiconductor film 110, the conductive film 114 overlapping with theoxide semiconductor film 110 with the insulating film 112 providedtherebetween, the insulating film 118 covering the oxide semiconductorfilm 110, the insulating film 112, and the conductive film 114, theinsulating film 120 over the insulating film 118, the conductive film122 connected to the oxide semiconductor film 110 through the openingportion 140 a provided in the insulating film 118 and the insulatingfilm 120, and the conductive film 124 connected to the oxidesemiconductor film 110 through the opening portion 140 b provided in theinsulating film 118 and the insulating film 120. Note that theinsulating film 128 covering the insulating film 120, the conductivefilm 122, and the conductive film 124 may be provided over thetransistor 100A.

Note that in FIG. 5C, the conductive film 106 has a stacked-layerstructure of a conductive film 106 a and a conductive film 106 b overthe conductive film 106 a. The insulating film 108 has a stacked-layerstructure of the insulating film 108 a and the insulating film 108 bover the insulating film 108 a. The conductive film 114 has astacked-layer structure of the conductive film 114 a and the conductivefilm 114 b over the conductive film 114 a. The conductive film 122 has astacked-layer structure of the conductive film 122 a and the conductivefilm 122 b over the conductive film 122 a. The conductive film 124 has astacked-layer structure of the conductive film 124 a and the conductivefilm 124 b over the conductive film 124 a.

In the transistor 100A, the conductive film 106 functions as a firstgate electrode (also referred to as a bottom-gate electrode), theconductive film 114 functions as a second gate electrode (also referredto as a top-gate electrode), the conductive film 122 functions as one ofa source electrode and a drain electrode, and the conductive film 124functions as the other of the source electrode and the drain electrode.Furthermore, in the transistor 100A, the insulating film 108 functionsas a first gate insulating film, and the insulating film 112 functionsas a second gate insulating film.

Note that the transistor 100A shown in FIGS. 5A and 5C is different fromthe transistor 100 described above and has a structure in which theconductive film functioning as a gate electrode is provided over andunder the oxide semiconductor film 110. As in the transistor 100A, twoor more gate electrodes may be provided in the semiconductor device ofone embodiment of the present invention.

The capacitor 150A illustrated in FIGS. 5B and 5D includes theinsulating film 104 formed over the substrate 102, the insulating film108 over the insulating film 104, the insulating film 112 over theinsulating film 108, the conductive film 116 over the insulating film112, the insulating film 118 covering the insulating film 108, theinsulating film 112, and the conductive film 116, the insulating film120 over the insulating film 118, and the conductive film 126overlapping with the conductive film 116 with the insulating film 118provided therebetween in the opening portion 140 c provided in theinsulating film 120. Note that the insulating film 128 covering theinsulating film 120 and the conductive film 126 may be provided over thecapacitor 150A.

Note that in FIG. 5D, the insulating film 108 has a stacked-layerstructure of the insulating film 108 a and the insulating film 108 bover the insulating film 108 a. The conductive film 116 has astacked-layer structure of the conductive film 116 a and the conductivefilm 116 b over the conductive film 116 a. The conductive film 126 has astacked-layer structure of the conductive film 126 a and the conductivefilm 126 b over the conductive film 126 a.

Furthermore, the capacitor 150A has a structure in which a dielectric isprovided between a pair of electrodes. In more detail, one of the pairof electrodes is the conductive film 116, the other of the pair ofelectrodes is the conductive film 126, and the insulating film 118between the conductive film 116 and the conductive film 126 functions asthe dielectric.

Note that the conductive film 114 functioning as the second gateelectrode of the transistor 100A and the conductive film 116 functioningas the one of the pair of electrodes of the capacitor 150A are formed inthe same step, and the conductive films 114 and 116 are at least partlyformed over the same surface. Furthermore, the conductive film 122 andthe conductive film 124 that function as the source electrode and thedrain electrode of the transistor 100A and the conductive film 126functioning as the other of the pair of electrodes of the capacitor 150Aare formed in the same step, and the conductive films 122, 124, and 126are at least partly formed over the same surface.

As described above, by forming the conductive films that function as theelectrodes of the transistor 100A and the capacitor 150A in the samestep, a manufacturing cost can be reduced.

Furthermore, in the capacitor 150A, the insulating film 120 has theopening portion 140 c. Therefore, in an insulating film in which theinsulating film 118 and the insulating film 120 are stacked, only theinsulating film 118 is made to function as the dielectric. The capacitor150A having such a structure can have a high capacitance value, andaccordingly, a display device can have a high capacitance value.

FIG. 6 is a cross-sectional view of the transistor 100A illustrated inFIG. 5A in the dashed-dotted line Y3-Y4 direction (the channel widthdirection).

As illustrated in FIG. 6, the conductive film 114 functioning as asecond gate electrode is connected to the conductive film 106functioning as a first gate electrode in an opening portion 139 providedin the insulating film 108 and the insulating film 112. Therefore, thesame potential is applied to the conductive film 114 and the conductivefilm 106. Note that the conductive film 114 and the conductive film 106are not necessarily connected to each other, in which case the openingportion 139 is not provided. In the case of employing the structure inwhich the conductive film 114 and the conductive film 106 are notconnected to each other, different potentials may be applied to theconductive film 114 and the conductive film 106.

Furthermore, as illustrated in FIG. 6, the oxide semiconductor film 110is positioned to face each of the conductive film 106 functioning as afirst gate electrode and the conductive film 114 functioning as a secondgate electrode, and is sandwiched between the two conductive filmsfunctioning as gate electrodes. The length in the channel widthdirection of the conductive film 114 functioning as a second gateelectrode is longer than the length in the channel width direction ofthe oxide semiconductor film 110. In the channel width direction, thewhole oxide semiconductor film 110 is covered with the conductive film114 with the insulating film 112 provided therebetween. Since theconductive film 114 functioning as a second gate electrode is connectedto the conductive film 106 functioning as a first gate electrode in theopening portion 139 provided in the insulating film 108 and theinsulating film 112, a side surface of the oxide semiconductor film 110in the channel width direction faces the conductive film 114 functioningas a second gate electrode with the insulating film 112 providedtherebetween.

In other words, in the channel width direction of the transistor 100A,the conductive film 106 functioning as a first gate electrode and theconductive film 114 functioning as a second gate electrode are connectedto each other in the opening portion provided in the insulating film 108functioning as a first gate insulating film and the insulating film 112functioning as a second gate insulating film; and the conductive film106 and the conductive film 114 surround the oxide semiconductor film110 with the insulating film 108 functioning as a first gate insulatingfilm and the insulating film 112 functioning as a second gate insulatingfilm provided therebetween.

Such a structure enables electric fields of the conductive film 106functioning as a first gate electrode and the conductive film 114functioning as a second gate electrode to electrically surround theoxide semiconductor film 110 included in the transistor 100A. A devicestructure of a transistor, like that of the transistor 100A, in whichelectric fields of a first gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film where a channel regionis formed can be referred to as a surrounded channel (s-channel)structure.

Since the transistor 100A has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 110 by the conductive film 106 functioning as a firstgate electrode or the conductive film 114 functioning as a second gateelectrode; therefore, the current drive capability of the transistor100A can be improved and high on-state current characteristics can beobtained. Since the on-state current can be increased, it is possible toreduce the size of the transistor 100A. In addition, since thetransistor 100A has a structure in which the oxide semiconductor film110 is surrounded by the conductive film 106 functioning as a first gateelectrode and the conductive film 114 functioning as a second gateelectrode, the mechanical strength of the transistor 100A can beincreased.

Note that in the channel width direction of the transistor 100A, anopening portion which is different from the opening portion 139 may beformed on the side of the oxide semiconductor film 110 where the openingportion 139 is not formed.

A material similar to the material of the insulating film 108 can beused for the insulating film 104 included in the transistor 100A and thecapacitor 150A. Here, a 100-nm-thick silicon nitride film is formedusing a PECVD apparatus as the insulating film 104.

A material similar to the material of each of the conductive films 114,122, and 124 can be used for the conductive film 106 included in thetransistor 100A. Here, a 10-nm-thick tantalum nitride film is formedusing a sputtering apparatus as the conductive film 106 a, and a300-nm-thick copper film is formed using a sputtering apparatus as theconductive film 106 b.

Next, another structure of the semiconductor devices illustrated inFIGS. 1A to 1D and FIGS. 5A to 5D is described with reference to FIGS.7A to 7D, FIGS. 8A to 8D, FIGS. 9A to 9D, FIG. 10, and FIG. 11A. Notethat the semiconductor devices illustrated in FIGS. 7A to 7D, FIGS. 8Ato 8D, FIGS. 9A to 9D, FIG. 10, and FIG. 11A are modification examplesof the semiconductor device illustrated in FIGS. 5A to 5D.

FIG. 7A is a cross-sectional view of a transistor 100B included in asemiconductor device. FIG. 7B is a cross-sectional view of a capacitor150B included in a semiconductor device. Note that top views of thetransistor 100B and the capacitor 150B are similar to the top viewsillustrated in FIGS. 5A and 5B; thus, they are not described here.Similarly, top views of a transistor 100C illustrated in FIG. 7C, acapacitor 150C illustrated in FIG. 7D, a transistor 100D illustrated inFIG. 8A, a capacitor 150D illustrated in FIG. 8B, a transistor 100Eillustrated in FIG. 8C, a capacitor 150E illustrated in FIG. 8D, atransistor 100F illustrated in FIG. 9A, a capacitor 150F illustrated inFIG. 9B, a transistor 1000 illustrated in FIG. 9C, and a capacitor 1500illustrated in FIG. 9D are similar to the top views illustrated in FIGS.5A and 5B; thus, they are not described here.

Furthermore, in the case where a portion illustrated in any of FIGS. 7Ato 7D, FIGS. 8A to 8D, FIGS. 9A to 9D, FIG. 10, and FIG. 11A has afunction similar to that described above, the same hatch pattern isapplied to the portion, and the portion is not especially denoted by areference numeral in some cases.

<Structure 3 of Semiconductor Device>

The transistor 100B illustrated in FIG. 7A differs from the transistor100A illustrated in FIG. 5C in the shape of the conductive film 114.Specifically, the conductive film 114 included in the transistor 100Bhas a stacked-layer structure of the conductive film 114 a and theconductive film 114 b over the conductive film 114 a, a lower endportion of the conductive film 114 a agrees with or substantially agreeswith an upper end portion of the insulating film 112, and a lower endportion of the conductive film 114 b is positioned on the inner sidethan an upper end portion of the conductive film 114 a. Furthermore, anend portion of the conductive film 114 b is partly arc-shaped.

The capacitor 150B illustrated in FIG. 7B differs from the capacitor150A illustrated in FIG. 5D in the shape of the conductive film 116.Specifically, the conductive film 116 included in the capacitor 150B hasa stacked-layer structure of the conductive film 116 a and theconductive film 116 b over the conductive film 116 a, a lower endportion of the conductive film 116 a agrees with or substantially agreeswith an upper end portion of the insulating film 112, and a lower endportion of the conductive film 116 b is positioned on the inner sidethan an upper end portion of the conductive film 116 a.

When the insulating film 112 and/or the conductive films 114 and 116have the shape illustrated in FIGS. 7A and 7B, the coverage with theinsulating film 118 can be increased.

<Structure 4 of Semiconductor Device>

The transistor 100C illustrated in FIG. 7C differs from the transistor100A illustrated in FIG. 5C in the shape of the insulating film 112.Specifically, a lower end portion and an upper end portion of theinsulating film 112 included in the transistor 100C are positioned onthe outer side than a lower end portion of the conductive film 114. Thatis, the insulating film 112 has a shape projecting from the conductivefilm 114. When the insulating film 112 has the shape illustrated in FIG.7C, the insulating film 118 can be kept away from the channel region ofthe oxide semiconductor film 110; thus, entry of nitrogen, hydrogen, andthe like contained in the insulating film 118 into the channel region ofthe oxide semiconductor film 110 can be suppressed.

The capacitor 150C illustrated in FIG. 7D differs from the capacitor150A illustrated in FIG. 5D in the shape of the insulating film 112.Specifically, a lower end portion and an upper end portion of theinsulating film 112 included in the capacitor 150C are positioned on theouter side than a lower end portion of the conductive film 116.

When the insulating film 112 has the shape illustrated in FIGS. 7C and7D, the coverage with the insulating film 118 can be increased.

<Structure 5 of Semiconductor Device>

The transistor 100D illustrated in FIG. 8A differs from the transistor100A illustrated in FIG. 5C in the structures of the insulating film 108and the insulating film 112. Specifically, the insulating film 108included in the transistor 100D illustrated in FIG. 8A has astacked-layer structure of the insulating film 108 a, the insulatingfilm 108 b, and an insulating film 108 c. The insulating film 112included in the transistor 100D shown in FIG. 8A has a stacked-layerstructure of an insulating film 112 a and an insulating film 112 b.

The capacitor 150D illustrated in FIG. 8B differs from the capacitor150A illustrated in FIG. 5D in the structures of the insulating film 108and the insulating film 112. Specifically, the insulating film 108included in the capacitor 150D illustrated in FIG. 8B has astacked-layer structure of the insulating film 108 a, the insulatingfilm 108 b, and the insulating film 108 c. The insulating film 112included in the capacitor 150D illustrated in FIG. 8B has astacked-layer structure of the insulating film 112 a and the insulatingfilm 112 b.

The insulating film 108 c and the insulating film 112 a can be formedusing an oxide insulating film having a low density of states ofnitrogen oxide. Note that the density of states of the nitrogen oxidecan be formed between the energy at the valence band maximum (E_(v_os))and the energy at the conduction band minimum (E_(c_os)) of the oxidesemiconductor film. A silicon oxynitride film that releases lessnitrogen oxide, an aluminum oxynitride film that releases less nitrogenoxide, or the like can be used as the oxide insulating film in which thedensity of states of nitrogen oxide is low between E_(v_os) andE_(e_os). Note that the average thickness of each of the insulatingfilms 108 c and 112 a is greater than or equal to 0.1 nm and less thanor equal to 50 nm, or greater than or equal to 0.5 nm and less than orequal to 10 nm.

Note that a silicon oxynitride film that releases less nitrogen oxide isa film of which the amount of released ammonia is larger than the amountof released nitrogen oxide in thermal desorption spectroscopy (TDS)analysis; the amount of released ammonia is typically greater than orequal to 1×10¹⁸ molecules/cm³ and less than or equal to 5×10¹⁹molecules/cm³. Note that the amount of released ammonia is the amount ofammonia released by heat treatment with which the surface temperature ofa film becomes higher than or equal to 50° C. and lower than or equal to650° C., preferably higher than or equal to 50° C. and lower than orequal to 550° C.

The insulating films 108 b and 112 b can be formed using an oxideinsulating film that releases oxygen by being heated. Note that theaverage thicknesses of the insulating films 108 b and 112 b are eachgreater than or equal to 5 nm and less than or equal to 1000 nm, orgreater than or equal to 10 nm and less than or equal to 500 am.

Typical examples of the oxide insulating film that releases oxygen bybeing heated include a silicon oxynitride film and an aluminumoxynitride film.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than orequal to 2, preferably greater than or equal to 1 and less than or equalto 2), typically NO₂ or NO, forms levels in the insulating film 108, theinsulating film 112, and the like. The level is positioned in the energygap of the oxide semiconductor film 110. Therefore, when nitrogen oxideis diffused to the interface between the insulating film 108 and theoxide semiconductor film 110, the interface between the insulating film112 and the oxide semiconductor film 110, and the interface between theinsulating film 108 and the insulating film 112, an electron is trappedby the level on the insulating film 108 side and the insulating film 112side. As a result, the trapped electron remains in the vicinity of theinterface between the insulating film 108 and the oxide semiconductorfilm 110, the interface between the insulating film 112 and the oxidesemiconductor film 110, and the interface between the insulating film108 and the insulating film 112; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide contained in the insulating films 108 b and 112 b reactswith ammonia contained in the insulating films 108 c and 112 a in heattreatment, nitrogen oxide contained in the insulating films 108 b and112 b is reduced. Therefore, an electron is hardly trapped at theinterface between the insulating film 108 and the oxide semiconductorfilm 110, the interface between the insulating film 112 and the oxidesemiconductor film 110, and the interface between the insulating film108 and the insulating film 112.

By using, for the insulating films 108 c and 112 a, the oxide insulatingfilm having a low density of states of nitrogen oxide between E_(v_os)and E_(c_os), the shift in the threshold voltage of the transistor canbe reduced, which leads to a smaller change in the electricalcharacteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating films108 and 112, by heat treatment of a manufacturing process of thetransistor, typically heat treatment at a temperature higher than orequal to 300° C. and lower than the strain point of the substrate, afirst signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, a second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and a third signal that appears at a g-factor of greater than orequal to 1.964 and less than or equal to 1.966 are observed. The splitwidth of the first and second signals and the split width of the secondand third signals that are obtained by ESR measurement using an X-bandare each approximately 5 mT. The sum of the spin densities of the firstsignal that appears at a g-factor of greater than or equal to 2.037 andless than or equal to 2.039, the second signal that appears at ag-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lowerthan 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x); x is greater than or equal to 0 and smaller than or equalto 2, preferably greater than or equal to 1 and less than or equal to2). Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

An oxide insulating film containing nitrogen and having a small amountof defects has a nitrogen concentration measured by SIMS of lower thanor equal to 6×10²⁰ atoms/cm³.

By forming an oxide insulating film containing nitrogen and having asmall amount of defects by a PECVD method using silane and dinitrogenmonoxide at a substrate temperature higher than or equal to 220° C.,higher than or equal to 280° C., or higher than or equal to 350° C., adense and hard film can be formed.

<Structure 6 of Semiconductor Device>

The transistor 100E illustrated in FIG. 8C differs from the transistor100A illustrated in FIG. 5C in the shapes of the insulating film 112 andthe conductive film 114. Specifically, an end portion of the insulatingfilm 112 included in the transistor 100E is partly arc-shaped.Furthermore, the lower end portion and the upper end portion of theconductive film 14 a are positioned on the inner side than the upper endportion of the insulating film 112. Furthermore, the lower end portionof the conductive film 114 b is positioned on the inner side than theupper end portion of the conductive film 114 a. Furthermore, the endportions of the conductive film 114 a and the conductive film 114 b arepartly arc-shaped.

The capacitor 150E illustrated in FIG. 8D differs from the capacitor150A illustrated in FIG. 5D in the shapes of the insulating film 112 andthe conductive film 116. Specifically, the end portion of the insulatingfilm 112 included in the capacitor 150E is partly arc-shaped.Furthermore, the lower end portion and the upper end portion of theconductive film 116 a are positioned on the inner side than the upperend portion of the insulating film 112. Furthermore, the lower endportion of the conductive film 116 b is positioned on the inner sidethan the upper end portion of the conductive film 116 a. Note that endportions of the conductive film 116 a and the conductive film 116 b arepartly arc-shaped.

<Structure 7 of Semiconductor Device>

The transistor 100F illustrated in FIG. 9A differs from the transistor100A illustrated in FIG. 5C in the shapes of the insulating film 112 andthe conductive film 114 and the like. Specifically, the insulating film112 and the conductive film 114 included in the transistor 100F have arectangular shape in a cross section. Furthermore, the transistor 100Fincludes an insulating film 117 between the oxide semiconductor film 110and the insulating film 118.

The capacitor 150F illustrated in FIG. 9B differs from the capacitor150A illustrated in FIG. 5D in the shapes of the insulating film 112 andthe conductive film 116 and the like. Specifically, the insulating film112 and the conductive film 116 included in the capacitor 150F have arectangular shape in a cross section. Furthermore, the capacitor 150Fincludes the insulating film 117 between the conductive film 116 and theinsulating film 118.

The insulating film 117 illustrated in FIGS. 9A and 9B can be formedusing an oxide insulating film containing nitrogen and having a smallamount of defects which can be used for the insulating film 108 c andthe insulating film 112 a in the transistor 100D and the capacitor 150Dwhich are illustrated in FIGS. 8A and 8B.

When the structure of the transistor 100F has the shape illustrated inFIG. 9A, the shapes of low-resistance regions formed in the oxidesemiconductor film 110 have a structure shown in FIG. 10.

FIG. 10 is an enlarged view of the vicinity of the oxide semiconductorfilm 110 of the transistor 100F illustrated in FIG. 9A. A region inwhich the carrier density of the oxide semiconductor film is increasedand the conductivity thereof is increased (low-resistance region) isformed in a cross section of the oxide semiconductor film 110 in thechannel length direction, as illustrated in FIG. 10. In FIG. 10, achannel length L corresponds to a length of a region between the pair oflow-resistance regions.

As illustrated in FIG. 10, in a cross-sectional shape in the channellength direction, the oxide semiconductor film 110 includes thelow-resistance region 110 d between the channel region 110 a and thelow-resistance region 110 b, and the low-resistance region 110 e betweenthe channel region 110 a and the low-resistance region 110 c. Thelow-resistance regions 110 d and 110 e have lower impurity elementconcentrations and higher resistivity than the low-resistance regions110 b and 110 c. Here, the low-resistance regions 110 d and 110 eoverlap with the insulating film 117 in contact with side surfaces ofthe insulating film 112 and the conductive film 114. Note that thelow-resistance regions 110 d and 110 e may overlap with the insulatingfilm 112 and the conductive film 114.

The oxide semiconductor film 110 includes the low-resistance regions 110d and 110 e having lower impurity element concentrations and higherresistivity than the low-resistance regions 110 b and 110 c, whereby theelectric field of the drain region can be relaxed. Thus, change in thethreshold voltage of the transistor due to the electric field of thedrain region can be reduced.

<Structure 8 of Semiconductor Device>

The transistor 100 illustrated in FIG. 9C differs from the transistor100A illustrated in FIG. 5C in the shapes of the insulating film 112 andthe oxide semiconductor film 110. Specifically, the insulating film 112included in the transistor 100G has two thicknesses; a thickness of aregion overlapping with the conductive film 114 is different from athickness of a region not overlapping with the conductive film 114. Thethickness of the region not overlapping with the conductive film 114 issmaller than the thickness of the region overlapping with the conductivefilm 114. Furthermore, the insulating film 112 covers the oxidesemiconductor film 110; therefore, the whole oxide semiconductor film110 has substantially the same thickness.

The capacitor 150G illustrated in FIG. 9D differs from the capacitor150A illustrated in FIG. 5D in the shape of the insulating film 112.Specifically, the insulating film 112 included in the capacitor 150G hastwo thicknesses; a thickness of a region overlapping with the conductivefilm 116 is different from a thickness of a region not overlapping withthe conductive film 116. The thickness of the region not overlappingwith the conductive film 116 is smaller than the thickness of the regionoverlapping with the conductive film 116.

For example, the insulating film 112 illustrated in FIGS. 9C and 9D canbe formed as follows: when the insulating film 112 is removed after theconductive film 114 is processed, a region of the insulating film 112that does not overlap with the conductive film 114 is left.

Note that in the transistor 100G illustrated in FIG. 9C, the insulatingfilm 112 is in contact with the channel region 110 a of the oxidesemiconductor film 110 and is in contact with the low-resistance regions110 b and 110 c. Furthermore, in the insulating film 112, thicknesses ofregions in contact with the low-resistance regions 110 b and 110 c aresmaller than a thickness of a region in contact with the channel region110 a; the average thickness of the insulating film 112 is typicallygreater than or equal to 0.1 nm and less than or equal to 50 nm, orgreater than or equal to 0.5 nm and less than or equal to 10 nm. As aresult, the impurity element can be added to the oxide semiconductorfilm 110 through the insulating film 112, and in addition, hydrogencontained in the insulating film 118 can be moved to the oxidesemiconductor film 110 through the insulating film 112. Thus, thelow-resistance regions 110 b and 110 c can be formed.

When the insulating film 112 is formed using an oxide insulating filmcontaining nitrogen and having a small amount of defects, nitrogen oxideis hardly generated in the insulating film 112, so that the carrier trapat the interface between the insulating film 112 and the oxidesemiconductor film 110 can be reduced. As a result, a shift in thethreshold voltage of each of the transistors can be reduced, which leadsto a smaller change in the electrical characteristics of thetransistors.

Furthermore, the insulating film 108 has a multilayer structure of theinsulating films 108 a, 108 b, and 108 c; for example, the insulatingfilm 108 a is formed using a nitride insulating film, the insulatingfilm 108 b is formed using an oxide insulating film that releases oxygenby being heated, and the insulating film 108 c is formed using an oxideinsulating film containing nitrogen and having a small amount ofdefects. Furthermore, the insulating film 112 is formed using an oxideinsulating film containing nitrogen and having a small amount ofdefects. That is, the oxide semiconductor film 110 can be covered withthe oxide insulating film containing nitrogen and having a small amountof defects. As a result, the carrier trap at the interfaces between theoxide semiconductor film 110 and the insulating films 108 c and 112 canbe reduced while oxygen contained in the insulating film 108 b is movedto the oxide semiconductor film 110 by heat treatment to reduce oxygenvacancies contained in the channel region 110 a of the oxidesemiconductor film 110. As a result, a shift in the threshold voltage ofthe transistor can be reduced, which leads to a smaller change in theelectrical characteristics of the transistor.

<Structure 9 of Semiconductor Device>

The transistor 100H illustrated in FIG. 11A differs from the transistor100A illustrated in FIG. 5C in the structure of the oxide semiconductorfilm 110. Specifically, the oxide semiconductor film 110 included in thetransistor 100H includes an oxide semiconductor film 110_1 and an oxidesemiconductor film 110_2 provided in contact with the oxidesemiconductor film 110_1. That is, the oxide semiconductor film 110 hasa multilayer structure.

Furthermore, the oxide semiconductor film 110 of the transistor 100Hillustrated in FIG. 11A includes the low-resistance regions describedabove. Specifically, the oxide semiconductor film 110 of the transistor100H includes a channel region 110 a_1, a channel region 110 a_2, alow-resistance region 110 b_, a low-resistance region 110 b_2, alow-resistance region 110 c_1, and a low-resistance region 110 c_2.

<Band Structure>

Here, a band structure in the A-B cross section including the channelregions of the transistor 100H is illustrated in FIG. 11B. Note that theoxide semiconductor film 110_2 is assumed to have a wider energy gapthan the oxide semiconductor film 110_1. Furthermore, the insulatingfilm 108 a, the insulating film 108 b, and the insulating film 112 areassumed to have wider energy gaps than the oxide semiconductor film110_1 and the oxide semiconductor film 110_2. Furthermore, the Fermilevels (denoted by Ef) of the oxide semiconductor film 110_1, the oxidesemiconductor film 110_2, the insulating film 108 a, the insulating film108 b, and the insulating film 112 are assumed to be equal to theintrinsic Fermi levels thereof (denoted by Ei). Furthermore, workfunctions of the conductive film 106 and the conductive film 114 areassumed to be equal to the Fermi levels.

When a gate voltage is set to be higher than or equal to the thresholdvoltage of the transistor, an electron flows preferentially in the oxidesemiconductor film 110_1 owing to the difference between the energies ofthe conduction band minimums of the oxide semiconductor film 110_1 andthe oxide semiconductor film 110_2. That is, it is probable that anelectron is embedded in the oxide semiconductor film 110_1. Note thatthe energy at the conduction band minimum is denoted by Ec, and theenergy at the valence band maximum is denoted by Ev.

Accordingly, in the transistor according to one embodiment of thepresent invention, the embedment of an electron reduces the influence ofinterface scattering. Therefore, the channel resistance of thetransistor according to one embodiment of the present invention is low.

Next, FIG. 11C shows a band structure in the C-D cross section includingthe source region or the drain region of the transistor. Note that thelow-resistance region 110 c_1 and the low-resistance region 110 c_2 areassumed to be in a degenerate state. Furthermore, the Fermi level of theoxide semiconductor film 110_1 is assumed to be approximately the sameas the energy of the conduction band minimum in the low-resistanceregion 110 c_1. Furthermore, the Fermi level of the oxide semiconductorfilm 110_2 is assumed to be approximately the same as the energy of theconduction band minimum in the low-resistance region 110 c_2.

At this time, an ohmic contact is made between the conductive film 124functioning as a source electrode or a drain electrode and thelow-resistance region 110 c_2 because an energy barrier therebetween issufficiently low. Furthermore, an ohmic contact is made between thelow-resistance region 110 c_2 and the low-resistance region 110 c_1.Therefore, electron transfer is conducted smoothly between theconductive film 124 and the oxide semiconductor films 110_1 and 110_2.

Note that description similar to that of FIG. 11C can be made on aregion where the conductive film 122 functioning as one of a sourceelectrode and a drain electrode of the transistor is in contact with thelow-resistance region 110 b_1 and the low-resistance region 110 b_2 ofthe oxide semiconductor film 110.

As described above, the transistor according to one embodiment of thepresent invention is a transistor in which the channel resistance is lowand electron transfer between the channel region and the source and thedrain electrodes is conducted smoothly. That is, the transistor hasexcellent switching characteristics.

<Connection Portions and Intersection Portion of Conductive Films ofSemiconductor Device>

Next, structures of connection portions and an intersection portion ofconductive films of the semiconductor device of one embodiment of thepresent invention illustrated in FIGS. 5A to 5D are described withreference to FIGS. 30A to 30D. Note that FIGS. 30A to 30C arecross-sectional views showing structures of connection portions ofconductive films, and FIG. 30D is a cross-sectional view showing astructure of an intersection portion of two different conductive films.

The connection portion illustrated in FIG. 30A includes the insulatingfilm 104 over the substrate 102, a conductive film 306 over theinsulating film 104, the insulating film 108 covering the conductivefilm 306, the insulating film 112 over the insulating film 108, aconductive film 314 which is provided over the insulating film 112 andis connected to the conductive film 306 in an opening portion 352provided in the insulating film 112 and the insulating film 108, theinsulating film 118 covering the insulating films 108 and 112 and theconductive film 314, the insulating film 120 over the insulating film118, a conductive film 318 which is provided over the insulating film120 and is connected to the conductive film 314 in an opening portion353 provided in the insulating films 118 and 120, and the insulatingfilm 128 covering the insulating film 120 and the conductive film 318.

The connection portion illustrated in FIG. 30B includes the insulatingfilm 104 over the substrate 102, the insulating film 108 over theinsulating film 104, the insulating film 112 over the insulating film108, a conductive film 324 over the insulating film 112, the insulatingfilm 118 covering the insulating films 108 and 112 and the conductivefilm 324, the insulating film 120 over the insulating film 118, aconductive film 328 which is provided over the insulating film 120 andis connected to the conductive film 324 in an opening portion 354provided in the insulating films 118 and 120, and the insulating film128 covering the insulating film 120 and the conductive film 328.

The connection portion illustrated in FIG. 30C includes the insulatingfilm 104 over the substrate 102, a conductive film 316 over theinsulating film 104, the insulating film 108 covering the conductivefilm 316, the insulating film 112 over the insulating film 108, aconductive film 334 which is provided over the insulating film 112 andis connected to the conductive film 316 in an opening portion 355provided in the insulating film 112 and the insulating film 108, theinsulating film 118 covering the insulating film 108 and the conductivefilm 334, the insulating film 120 over the insulating film 118, and theinsulating film 128 over the insulating film 120.

The intersection portion illustrated in FIG. 30D includes the insulatingfilm 104 over the substrate 102, a conductive film 326 over theinsulating film 104, the insulating film 108 covering the conductivefilm 326, the insulating film 118 over the insulating film 108, theinsulating film 120 over the insulating film 118, a conductive film 338over the insulating film 120, and the insulating film 128 over theconductive film 338.

Note that in FIGS. 30A to 30D, the insulating film 108 has astacked-layer structure of the insulating film 108 a and the insulatingfilm 108 b over the insulating film 108 a. Furthermore, in FIG. 30A, theconductive film 306 has a stacked-layer structure of a conductive film306 a and a conductive film 306 b over the conductive film 306 a, theconductive film 314 has a stacked-layer structure of a conductive film314 a and a conductive film 314 b over the conductive film 314 a, andthe conductive film 318 has a stacked-layer structure of a conductivefilm 318 a and a conductive film 318 b over the conductive film 318 a.Furthermore, in FIG. 30B, the conductive film 324 has a stacked-layerstructure of a conductive film 324 a and a conductive film 324 b overthe conductive film 324 a, and the conductive film 328 has astacked-layer structure of a conductive film 328 a and a conductive film328 b over the conductive film 328 a. Furthermore, in FIG. 30C, theconductive film 316 has a stacked-layer structure of a conductive film316 a and a conductive film 316 b over the conductive film 316 a, andthe conductive film 334 has a stacked-layer structure of a conductivefilm 334 a and a conductive film 334 b over the conductive film 334 a.Furthermore, in FIG. 30D, the conductive film 326 has a stacked-layerstructure of a conductive film 326 a and a conductive film 326 b overthe conductive film 326 a, and the conductive film 338 has astacked-layer structure of a conductive film 338 a and a conductive film338 b over the conductive film 338 a.

The conductive films 306, 316, and 326 are formed in the same step as astep of forming the conductive film 106 included in the transistor 100A.That is, the conductive film 106, the conductive film 306, theconductive film 316, and the conductive film 326 are at least partlyformed over the same surface. Furthermore, the conductive films 314,324, and 334 are formed in the same step as a step of forming theconductive film 114 included in the transistor 100A and the conductivefilm 116 included in the capacitor 150A. That is, the conductive film114, the conductive film 116, the conductive film 314, the conductivefilm 324, and the conductive film 334 are at least partly formed overthe same surface. Furthermore, the conductive films 318, 328, and 338are formed in the same step as a step of forming the conductive films122 and 124 included in the transistor 100A and the conductive film 126included in the capacitor 150A. That is, the conductive film 124, theconductive film 126, the conductive film 318, the conductive film 328,and the conductive film 338 are at least partly formed over the samesurface.

Furthermore, as illustrated in FIG. 30D, the insulating film 108, theinsulating film 118, and the insulating film 120 are provided betweenthe conductive film 326 and the conductive film 338. That is, theintersection portion of the conductive film 326 and the conductive film338 has a structure in which a plurality of insulating films is stacked.When the intersection portion of the conductive films has the structureas illustrated in FIG. 30D, parasitic capacitance in a portion where theconductive films intersect each other can be reduced. As a result,signal delay due to the parasitic capacitance can be reduced.

<Method 1 for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the transistor 100 andthe capacitor 150 in FIGS. 1A to 1D is described with reference to FIGS.12A to 12H, FIGS. 13A to 13F, FIGS. 14A to 14F, FIGS. 15A to 15F, andFIGS. 16A to 16F.

Note that the films included in the transistor 100 and the capacitor 150(i.e., the insulating film, the oxide semiconductor film, the conductivefilm, and the like) can be formed by any of a sputtering method, achemical vapor deposition (CVD) method, a vacuum evaporation method, anda pulsed laser deposition (PLD) method. Alternatively, a coating methodor a printing method can be used. Although the sputtering method and aplasma-enhanced chemical vapor deposition (PECVD) method are typicalexamples of the film formation method, a thermal CVD method may be used.As the thermal CVD method, a metal organic chemical vapor deposition(MOCVD) method or an atomic layer deposition (ALD) method may be used,for example.

Deposition by the thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). In such a case, a first source gas is introduced,an inert gas (e.g., argon or nitrogen) or the like is introduced at thesame time or after the first source gas is introduced so that the sourcegases are not mixed, and then a second source gas is introduced. Notethat in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a firstsingle-atomic layer; then the second source gas is introduced to reactwith the first single-atomic layer, as a result, a second single-atomiclayer is stacked over the first single-atomic layer, so that a thin filmis formed.

The sequence of the gas introduction is repeated plural times until adesired thickness is obtained, whereby a thin film with excellent stepcoverage can be formed. The thickness of the thin film can be adjustedby the number of repetition times of the sequence of the gasintroduction; therefore, an ALD method makes it possible to accuratelyadjust a thickness and thus is suitable for manufacturing a minute FET.

Note that FIGS. 12A, 12C, 12E, and 12G, FIGS. 13A, 13C, and 13E, FIGS.14A, 14C, and 14E, FIGS. 15A, 15C, and 15E, and FIGS. 16A, 16C, and 16Eare cross-sectional views illustrating a method for manufacturing thetransistor 100, and FIGS. 12B, 12D, 12F, and 12H, FIGS. 13B, 13D, and13F, FIGS. 14B, 14D, and 14F, FIGS. 15B, 15D, and 15F, and FIGS. 16B,16D, and 16F are cross-sectional views illustrating a method formanufacturing the capacitor 150.

First, the insulating film 108 (the insulating film 108 a and theinsulating film 108 b) is formed over the substrate 102 (see FIGS. 12Aand 12B).

The insulating film 108 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. In thisembodiment, a 100-nm-thick silicon nitride film is formed using a PECVDapparatus as the insulating film 108 a. Furthermore, a 400-nm-thicksilicon oxynitride film is formed using a PECVD apparatus as theinsulating film 108 b.

After the insulating film 108 b is formed, oxygen may be added to theinsulating film 108 b. Examples of oxygen added to the insulating film108 b include an oxygen radical, an oxygen atom, an oxygen atomic ion,and an oxygen molecular ion. As a method for adding the oxygen, an iondoping method, an ion implantation method, plasma treatment, or the likecan be given. Alternatively, after a film that suppresses release ofoxygen is formed over the insulating film, oxygen may be added to theinsulating film 108 b through the film.

Alternatively, as the insulating film 108 b, a silicon oxide film or asilicon oxynitride film that can release oxygen by heat treatment can beformed under the following conditions: the substrate placed in atreatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature higher than or equal to 180° C. and lower than orequal to 280° C., or higher than or equal to 200° C. and lower than orequal to 240° C., the pressure is greater than or equal to 100 Pa andless than or equal to 250 Pa, or greater than or equal to 100 Pa andless than or equal to 200 Pa with introduction of a source gas into thetreatment chamber, and a high-frequency power of greater than or equalto 0.17 W/cm² and less than or equal to 0.5 W/cm², or greater than orequal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied toan electrode provided in the treatment chamber.

Here, a method in which a film that suppresses release of oxygen isformed over the insulating film 108 b and then oxygen is added to theinsulating film 108 b through the film is described.

A film 141 that suppresses release of oxygen is formed over theinsulating film 108 b (see FIGS. 12C and 12D).

Next, oxygen 142 is added to the insulating film 108 h through the film141 (see FIGS. 12E and 12F).

The film 141 that suppresses release of oxygen is formed using any ofthe following conductive materials: a metal element selected fromaluminum, chromium, tantalum, titanium, molybdenum, nickel, iron,cobalt, and tungsten; an alloy containing the above-described metalelement as a component; an alloy containing any of the above-describedmetal elements in combination; a metal nitride containing theabove-described metal element; a metal oxide containing theabove-described metal element; a metal nitride oxide containing theabove-described metal element; and the like.

The thickness of the film 141 that suppresses release of oxygen can begreater than or equal to 1 nm and less than or equal to 20 nm, orgreater than or equal to 2 nm and less than or equal to 10 nm.

As a method for adding the oxygen 142 to the insulating film 108 bthrough the film 141, an ion doping method, an ion implantation method,plasma treatment, or the like is given. By adding oxygen to theinsulating film 108 b with the film 141 provided over the insulatingfilm 108 b, the film 141 functions as a protective film that suppressesrelease of oxygen from the insulating film 108 b. Thus, more oxygen canbe added to the insulating film 108 b.

In the case where oxygen is added by plasma treatment, by making oxygenexcited by a microwave to generate high-density oxygen plasma, theamount of oxygen added to the insulating film 108 b can be increased.

Then, the film 141 is removed (see FIGS. 120 and 12H).

Note that the treatment for adding oxygen which is illustrated in FIGS.12C and 12D and FIGS. 12E and 12F is not necessarily performed in thecase where the insulating film 108 b to which a sufficient amount ofoxygen is added can be formed after its deposition.

Next, an oxide semiconductor film is formed over the insulating film 108b, and the oxide semiconductor film is processed into a desired shape,whereby the oxide semiconductor film 110 is formed. After that, theinsulating film 112 is formed over the insulating film 108 b and theoxide semiconductor film 110 (see FIGS. 13A and 13B).

A formation method of the oxide semiconductor film 110 is describedbelow. An oxide semiconductor film is formed over the insulating film108 b by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, a thermal CVD method, or thelike. Then, after a mask is formed over the oxide semiconductor film bya lithography step, the oxide semiconductor film is partly etched usingthe mask. Accordingly, the oxide semiconductor film 110 can be formed asillustrated in FIG. 13A. After that, the mask is removed. Note that heattreatment may be performed after the oxide semiconductor film 110 isformed.

Alternatively, by using a printing method for forming the oxidesemiconductor film 110, the oxide semiconductor film 110 subjected toelement isolation can be formed directly.

As a power supply device for generating plasma in the case of formingthe oxide semiconductor film by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate. Note that a CAAC-OS film can be formed usingan AC power supply device or a DC power supply device. In forming theoxide semiconductor film, a sputtering method using an AC power supplydevice or a DC power supply device is preferable to a sputtering methodusing an RF power supply device because the oxide semiconductor film canbe uniform in film thickness, film composition, or crystallinity.

In the case where the oxide semiconductor film is formed by a sputteringmethod, as a sputtering gas, a rare gas (typically argon), an oxygengas, or a mixed gas of a rare gas and an oxygen gas is used asappropriate. In the case of using the mixed gas of a rare gas and anoxygen gas, the proportion of oxygen to a rare gas is preferablyincreased.

Furthermore, in the case where the oxide semiconductor film is formed bya sputtering method, a sputtering target may be appropriately selectedin accordance with the composition of the oxide semiconductor film to beformed.

S Note that in the case where the oxide semiconductor film is formed by,for example, a sputtering method at a substrate temperature higher thanor equal to 150° C. and lower than or equal to 750° C., higher than orequal to 150° C. and lower than or equal to 450° C., or higher than orequal to 200° C. and lower than or equal to 350° C. to deposit an oxidesemiconductor film, a CAAC-OS film can be formed. In the case where thesubstrate temperature is higher than or equal to 25° C. and lower than150° C., a microcrystalline oxide semiconductor film can be formed.

For the deposition of the CAAC-OS film to be described later, thefollowing conditions are preferably used.

By suppressing entry of impurities into the CAAC-OS film during thedeposition, the crystal state can be prevented from being broken by theimpurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in thedeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, or −100° C. or loweris used.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is set to be higher than or equal to 30 vol. %, or is setto be 100 vol. %.

After the oxide semiconductor film is formed, dehydrogenation ordehydration may be performed by heat treatment. The heat treatment isperformed typically at a temperature higher than or equal to 150° C. andlower than the strain point of the substrate, higher than or equal to250° C. and lower than or equal to 450° C., or higher than or equal to300° C. and lower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containingnitrogen or a rare gas such as helium, neon, argon, xenon, or krypton.The heat treatment may be performed under an inert gas atmosphere first,and then under an oxygen atmosphere. It is preferable that the aboveinert gas atmosphere and the above oxygen atmosphere do not containhydrogen, water, and the like. The treatment time is from 3 minutes to24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By depositing the oxide semiconductor film while it is heated orperforming heat treatment after the formation of the oxide semiconductorfilm, the hydrogen concentration in the oxide semiconductor film whichis measured by SIMS can be 5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³or lower, 5×10¹⁸ atoms/cm³ or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷atoms/cm³ or lower, or 1×10¹⁶ atoms/cm³ or lower.

For example, in the case where an oxide semiconductor film, e.g., anInGaZnO_(X) (X>0) film is deposited using a deposition apparatusemploying ALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introducedplural times to form an InO₂ layer, a Ga(CH₃), gas and an O₃ gas areintroduced at a time to form a GaO layer, and then a Zn(CH₃)₂ gas and anO₃ gas are introduced at a time to form a ZnO layer. Note that the orderof these layers is not limited to this example. A mixed compound layersuch as an InGaO₂ layer, an InZnO₂ layer, a GaInO layer, a ZnInO layer,or a GaZnO layer may be formed by mixing of these gases. Note thatalthough an H₂O gas which is obtained by bubbling with an inert gas suchas Ar may be used instead of an Os gas, it is preferable to use an O₃gas, which does not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used.Furthermore, a Zn(CH₃)₂ gas may be used.

Note that in this embodiment, the oxide semiconductor film 110 is formedas follows. A 50-nm-thick oxide semiconductor film is deposited using asputtering apparatus and using an In—Ga—Zn metal oxide (In:Ga:Zn=1:1:1.2[atomic ratio]) as a sputtering target, and then, heat treatment isperformed, whereby oxygen contained in the insulating film 108 b ismoved to the oxide semiconductor film. Next, a mask is formed over theoxide semiconductor film, and part of the oxide semiconductor film isselectively etched. Thus, the oxide semiconductor film 110 is formed.

When the heat treatment is performed at a temperature higher than 350°C. and lower than or equal to 650° C., or higher than or equal to 450°C. and lower than or equal to 600° C., it is possible to obtain an oxidesemiconductor film whose proportion of CAAC, which is described later,is greater than or equal to 60% and less than 100%, greater than orequal to 80% and less than 100%, greater than or equal to 90% and lessthan 100%, or greater than or equal to 95% and less than or equal to98%. Furthermore, it is possible to obtain an oxide semiconductor filmhaving a low content of hydrogen, water, and the like. That is, an oxidesemiconductor film with a low impurity concentration and a low densityof defect states can be formed.

The insulating film 112 can be formed by the formation method of theinsulating film 108 b as appropriate. As the insulating film 112, asilicon oxide film or a silicon oxynitride film can be formed by a PECVDmethod. In this case, a deposition gas containing silicon and anoxidizing gas are preferably used as a source gas. Typical examples ofthe deposition gas containing silicon include silane, disilane,trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone,dinitrogen monoxide, and nitrogen dioxide can be given as examples.

The silicon oxynitride film having a small amount of defects can beformed as the insulating film 112 by a PECVD method under the conditionswhere the ratio of an oxidizing gas to a deposition gas is higher than20 times and lower than 100 times or higher than or equal to 40 timesand lower than or equal to 80 times and the pressure in a treatmentchamber is lower than 100 Pa or lower than or equal to 50 Pa.

As the insulating film 112, a silicon oxide film or a silicon oxynitridefilm which is dense can be formed under the following conditions: thesubstrate placed in a treatment chamber of a PECVD apparatus that isvacuum-evacuated is held at a temperature higher than or equal to 280°C. and lower than or equal to 400° C., the pressure is greater than orequal to 20 Pa and less than or equal to 250 Pa, preferably greater thanor equal to 100 Pa and less than or equal to 250 Pa with introduction ofa source gas into the treatment chamber, and a high-frequency power issupplied to an electrode provided in the treatment chamber.

The insulating film 112 can be formed by a plasma CVD method using amicrowave. The microwave refers to a wave in the frequency range of 300MHz to 300 GHz. In the case of a microwave, electron temperature is lowand electron energy is low. Further, in supplied power, the proportionof power used for acceleration of electrons is low, and therefore, powercan be used for dissociation and ionization of more molecules. Thus,plasma with high density (high-density plasma) can be excited.Therefore, a deposition surface and a deposit are less damaged byplasma, and the insulating film 112 with few defects can be formed.

Alternatively, the insulating film 112 can be formed by a CVD methodusing an organosilane gas. As the organosilane gas, any of the followingsilicon-containing compound can be used: tetraethyl orthosilicate (TEOS)(chemical formula: Si(OC₂H₅)₄); tetramethylsilane (TMS) (chemicalformula: Si(CH₃)₄); tetramethylcyclotetrasiloxane (TMCTS);octamethylcyclotetrasiloxane (OMCTS); hexamethyldisilazane (HMDS);triethoxysilane (SiH(OC₂H₅)₃); trisdimethylaminosilane (SiH(N(CH₃)₂)₃);or the like. By a CVD method using the organosilane gas, the insulatingfilm 112 having high coverage can be formed.

In the case where a gallium oxide film is formed as the insulating film112, metal organic chemical vapor deposition (MOCVD) can be used.

In the case where a hafnium oxide film is formed as the insulating film112 by a thermal CVD method such as an MOCVD method or an ALD method,two kinds of gases, i.e. ozone (O₃) as an oxidizer and a source gas thatis obtained by vaporizing liquid containing a solvent and a hafniumprecursor compound (a hafnium alkoxide solution, typically tetrakis(dimethylamide) hafnium (TDMAH)) are used. Note that the chemicalformula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples ofanother material liquid include tetrakis(ethylmethylamide)hafnium.

In the case where an aluminum oxide film is formed as the insulatingfilm 112 by a thermal CVD method such as an MOCVD method or an ALDmethod, two kinds of gases, e.g., H₂O as an oxidizer and a source gasthat is obtained by vaporizing liquid containing a solvent and analuminum precursor compound (e.g., trimethylaluninum (TMA)) are used.Note that the chemical formula of trimethylaluminum is Al(CH₃). Examplesof another material liquid include tris(dimethylamide)aluminum,triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate). Note that the ALD methodenables the insulating film 112 to have excellent coverage and smallthickness.

In the case where a silicon oxide film is formed as the insulating film112 by a thermal CVD method such as an MOCVD method or an ALD method,hexachlorodisilane is adsorbed on a deposition surface, chlorinecontained in adsorbate is removed, and radicals of an oxidizing gas(e.g., O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

Here, a 100-nm-thick silicon oxynitride film is formed using a PECVDapparatus as the insulating film 112.

Next, a conductive film 113 (including a conductive film 113 a and aconductive film 113 b) is formed over the insulating film 112 (see FIGS.13C and 13D).

The conductive film 113 can be formed by a sputtering method, a vacuumevaporation method, a pulsed laser deposition (PLD) method, a thermalCVD method, or the like. In this embodiment, a 10-nm-thick tantalumnitride film is formed using a sputtering apparatus as the conductivefilm 113 a. Furthermore, a 300-nm-thick copper film is formed using asputtering apparatus as the conductive film 113 b. Note that thesuccessive formation of the conductive film 113 a and the conductivefilm 113 b in a vacuum is preferable because entry of impurities into aninterface between the conductive film 113 a and the conductive film 113b can be suppressed.

Alternatively, a tungsten film can be formed as the conductive film 113b with a deposition apparatus employing an ALD method. In that case, aWF₆ gas and a B₂H₆ gas are sequentially introduced more than once toform an initial tungsten film, and then a WF₆ gas and an H₂ gas areintroduced at a time, so that a tungsten film is formed. Note that anSiH₄ gas may be used instead of a B₂H₆ gas.

Next, a mask 145 is formed over the conductive film 113 b by alithography step, and then, the conductive film 113 b, the conductivefilm 113 a, and the insulating film 112 are partly etched (see FIGS. 13Eand 13F).

As a method for etching the conductive film 113 and the insulating film112, a wet etching method or/and a dry etching method can be used asappropriate.

Next, the conductive film 113 and the insulating film 112 are processedwhile the mask 145 is reduced, whereby the conductive films 114 a, 114b, 116 a, and 116 b are formed (see FIGS. 14A and 14B).

In the transistor 100, the oxide semiconductor film 110 is partlyexposed in a step of etching the conductive film 113 and the insulatingfilm 112. Note that a region where part of the oxide semiconductor film110 is exposed has a smaller thickness than the oxide semiconductor film110 overlapping with the conductive film 114 by a step of etching theconductive film 114 and the insulating film 112, in some cases.Furthermore, in the transistor 100, a region of the insulating film 108b functioning as a base film which is exposed from the oxidesemiconductor film 110 is partly removed in a step of etching theconductive film 113 and the insulating film 112, and thus, the thicknessof the region is smaller than that of a region overlapping with theoxide semiconductor film 110 in some cases. Furthermore, in thecapacitor 150, a region of the insulating film 108 b functioning as abase film which is exposed from the insulating film 112 is partlyremoved in a step of etching the conductive film 113 and the insulatingfilm 112, and thus, the thickness of the region is smaller than that ofa region overlapping with the insulating film 112 in some cases.

Next, an impurity element 143 is added over the insulating film 108 b,the insulating film 112, the oxide semiconductor film 110, theconductive film 114, and the mask 145 (see FIGS. 14C and 14D).

In a step of adding the impurity element 143, the impurity element isadded to regions of the oxide semiconductor film 110 which are notcovered with the conductive film 114, the insulating film 112, and themask 145. Note that an oxygen vacancy is formed in the oxidesemiconductor film 110 by the addition of the impurity element 143.

As a method for adding the impurity element 143, an ion doping method,an ion implantation method, plasma treatment, or the like can be given.In the case of plasma treatment, plasma is generated in a gas atmospherecontaining an impurity element to be added and plasma treatment isperformed, whereby the impurity element can be added. A dry etchingapparatus, an ashing apparatus, a plasma CVD apparatus, a high-densityplasma CVD apparatus, or the like can be used to generate the plasma.

Note that, as a source gas of the impurity element 143, one or more ofB₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, H₂, and arare gas can be used. Alternatively, one or more of B₂H₆, PH₃, N₂, NH₃,AlH₃, AlCl₃, F₂, HF, and H₂ which are diluted with a rare gas can beused. By adding the impurity element 143 to the oxide semiconductor film110 using one or more of B₂H₆, PH₃, N₂, NH₃, AlH₃, AlCl₃, F₂, HF, and H₂which are diluted with a rare gas, the rare gas and one or more ofhydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon,phosphorus, and chlorine can be added at a time to the oxidesemiconductor film 110.

Alternatively, after a rare gas is added to the oxide semiconductor film110, one or more of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆,F₂, HF, and H₂ may be added to the oxide semiconductor film 110.

Alternatively, after one or more of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃,AlCl₃, SiH₄, Si₂H₆, F₂, HF, and H₂ are added to the oxide semiconductorfilm 110, a rare gas may be added to the oxide semiconductor film 110.

The addition of the impurity element 143 is controlled by appropriatelysetting the implantation conditions such as the acceleration voltage andthe dose. For example, in the case where argon is added by an ionimplantation method, the acceleration voltage may be set to 10 kV andthe dose may be set to greater than or equal to 1×10¹³ ions/cm² and lessthan or equal to 1×10¹⁶ ions/cm², for example, 1×10¹⁴ ions/cm². In thecase where a phosphorus ion is added by an ion implantation method, theacceleration voltage may be set to 30 kV and the dose may be set togreater than or equal to 1×10¹³ ions/cm² and less than or equal to5×10¹⁶ ions/cm², for example, 1×10¹⁵ ions/cm².

In the case where argon is added as the impurity element 143 using a dryetching apparatus, the substrate may be set to a parallel plate on thecathode side and an RF power may be supplied so that a bias is appliedto the substrate side. As the RF power, for example, power density canbe greater than or equal to 0.1 W/cm² and less than or equal to 2 W/cm².

It is preferable that the impurity element 143 be added in a state wherethe mask 145 is left as in this embodiment. By the addition of theimpurity element 143 in a state where the mask 145 is left, adhesion ofa constituent element of the conductive film 114 to a sidewall of theinsulating film 112 can be suppressed. However, a method for adding theimpurity element 143 is not limited thereto; for example, the impurityelement 143 may be added using the conductive film 114 and theinsulating film 112 as masks after the mask 145 is removed.

After that, heat treatment may be performed to further increase theconductivity of the region to which the impurity element 143 is added.The heat treatment is performed typically at a temperature higher thanor equal to 150° C. and lower than the strain point of the substrate,higher than or equal to 250° C. and lower than or equal to 450° C., orhigher than or equal to 300° C. and lower than or equal to 450° C.

Next, the mask 145 is removed (see FIGS. 14E and 14F).

Next, the insulating film 118 is formed over the insulating film 108 b,the oxide semiconductor film 110, and the conductive films 114 and 116,and the insulating film 120 is formed over the insulating film 118 (seeFIGS. 15A and 15B).

For formation of the insulating film 118 and the insulating film 120,the formation method of the insulating film 108 a and the insulatingfilm 108 b can be used as appropriate.

In this embodiment, a 100-nm-thick silicon nitride film is formed usinga PECVD apparatus as the insulating film 118. Furthermore, a300-nm-thick silicon oxynitride film is formed using a PECVD apparatusas the insulating film 120.

When the insulating film 118 is formed of a silicon nitride film,hydrogen in the silicon nitride film enters the oxide semiconductor film110, so that the concentration of carriers in a region of the oxidesemiconductor film 110 in contact with the insulating film 118 can befurther increased.

Next, a mask is formed over the insulating film 120 by a lithographystep, and then, the insulating film 120 is partly etched, whereby theopening portion 140 c that reaches the insulating film 118 is formed(see FIGS. 15C and 15D).

As a method for etching the insulating film 120, a wet etching methodor/and a dry etching method can be used as appropriate.

Next, a mask is formed over the insulating film 120 by a lithographystep, and then, the insulating film 118 and the insulating film 120 arepartly etched, whereby the opening portion 140 a and the opening portion140 b that reach the oxide semiconductor film 110 is formed (see FIGS.15E and 15F).

Note that in this embodiment, the opening portion 140 c is formed in astep different from the step in which the opening portion 140 a and theopening portion 140 b are formed; however, a formation method of theopening portions is not limited thereto. For example, the openingportion 140 c, the opening portion 140 a, and the opening portion 140 bmay be formed at a time using a half-tone mask or a gray-tone mask.Using the half-tone mask or the gray-tone mask can reduce onelithography step, which leads to a reduction in a manufacturing cost.

Next, a conductive film 121 (including a conductive film 121 a and aconductive film 121 b) is formed over the insulating film 120 to coverthe opening portion 140 a, the opening portion 140 b, and the openingportion 140 c (see FIGS. 16A and 16B).

The conductive film 121 can be formed by the formation method of theconductive film 113 as appropriate. Here, a 50-nm-thick tungsten film isformed using a sputtering apparatus as the conductive film 121 a.Furthermore, a 200-nm-thick copper film is formed using a sputteringapparatus as the conductive film 121 b.

Next, a mask is formed over the conductive film 121 b by a lithographystep, and then, the conductive film 121 a and the conductive film 121 bare partly etched, whereby the conductive film 122, the conductive film124, and the conductive film 126 are formed (see FIGS. 16C and 16D).

Note that the conductive film 122 has a stacked-layer structure of theconductive film 122 a and the conductive film 122 b over the conductivefilm 122 a. Furthermore, the conductive film 124 has a stacked-layerstructure of the conductive film 124 a and the conductive film 124 bover the conductive film 124 a. Furthermore, the conductive film 126 hasa stacked-layer structure of the conductive film 126 a and theconductive film 126 b over the conductive film 126 a.

Next, the insulating film 128 is formed over the insulating film 120,the conductive film 122, the conductive film 124, and the conductivefilm 126 (see FIGS. 16E and 16F).

The insulating film 128 can be formed by the formation method of theinsulating film 108 a as appropriate. Here, a 200-nm-thick siliconnitride film is formed using a PECVD apparatus as the insulating film128.

Through the above-described process, the transistor 100 and thecapacitor 150 can be manufactured over the same substrate.

<Method 2 for Manufacturing Semiconductor Device>

Next, an example of a method for manufacturing the transistor 100A andthe capacitor 150A in FIGS. 5A to 5D is described below.

The insulating film 104 is formed over the substrate 102. Next, aconductive film is formed over the insulating film 104, and theconductive film is processed into a desired shape, whereby theconductive film 106 is formed. Next, steps similar to those illustratedin FIGS. 12A to 12H and FIGS. 13A and 13B are performed. After that, amask is formed over the insulating film 112 by a lithography step, andthen, the insulating film 112 is partly etched, whereby the openingportion 139 that reaches the conductive film 106 is formed. Stepsfollowing this can be performed in manners similar to those of the stepsillustrated in FIG. 13C and subsequent figures. Thus, the transistor100A and the capacitor 150A illustrated in FIGS. 5A to 5D can bemanufactured over the same substrate.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2

In this embodiment, the structure of an oxide semiconductor filmincluded in a semiconductor device of one embodiment of the presentinvention is described below in detail.

First, a structure which can be included in an oxide semiconductor filmis described below.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

First, a CAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflecting a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged parallel to the formation surfaceor the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

FIG. 17A is a cross-sectional TEM image of a CAAC-OS film. FIG. 17B is across-sectional TEM image obtained by enlarging the image of FIG. 17A.In FIG. 17B, atomic arrangement is highlighted for easy understanding.

FIG. 17C is local Fourier transform images of regions each surrounded bya circle (the diameter is about 4 nm) between A and O and between O andA′ in FIG. 17A. C-axis alignment can be observed in each region in FIG.17C. The c-axis direction between A and O is different from that betweenO and A′, which indicates that a grain in the region between A and O isdifferent from that between O and A′. In addition, between A and O, theangle of the c-axis gradually and continuously changes from 14.3°, 16.6°to 30.9°. Similarly, between O and A′, the angle of the c-axis graduallyand continuously changes from −18.3°, −17.6°, to −11.3°.

Note that in an electron diffraction pattern of the CAAC-OS film, spots(bright spots) having alignment are shown. For example, when electrondiffraction with an electron beam having a diameter of 1 nm or more and30 nm or less (such electron diffraction is also referred to as nanobeamelectron diffraction) is performed on the top surface of the CAAC-OSfilm, spots are observed (see FIG. 18A).

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

Most of the crystal parts included in the CAAC-OS film each fit inside acube whose one side is less than 100 nm. Thus, there is a case where acrystal part included in the CAAC-OS film fits inside a cube whose oneside is less than 10 nm, less than 5 nm, or less than 3 nm. Note thatwhen a plurality of crystal parts included in the CAAC-OS film areconnected to each other, one large crystal region is formed in somecases. For example, a crystal region with an area of 2500 nm² or more, 5μm² or more, or 1000 μm² or more is observed in some cases in the planTEM image.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (ϕ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (ϕaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when 0 scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are irregularlyoriented between crystal parts, the c-axes are aligned in a directionparallel to a normal vector of a formation surface or a normal vector ofa top surface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned in adirection parallel to a normal vector of a formation surface or a normalvector of a top surface. Thus, for example, in the case where a shape ofthe CAAC-OS film is changed by etching or the like, the c-axis might notbe necessarily parallel to a normal vector of a formation surface or anormal vector of a top surface of the CAAC-OS film.

Further, distribution of c-axis aligned crystal parts in the CAAC-OSfilm is not necessarily uniform. For example, in the case where crystalgrowth leading to the crystal parts of the CAAC-OS film occurs from thevicinity of the top surface of the film, the proportion of the c-axisaligned crystal parts in the vicinity of the top surface is higher thanthat in the vicinity of the formation surface in some cases. Further,when an impurity is added to the CAAC-OS film, a region to which theimpurity is added is altered, and the proportion of the c-axis alignedcrystal parts in the CAAC-OS film varies depending on regions, in somecases.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, a crystal part in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. Amicrocrystal with a size greater than or equal to 1 nm and less than orequal to 10 nm, or a size greater than or equal to 1 nm and less than orequal to 3 nm is specifically referred to as nanocrystal (nc). An oxidesemiconductor film including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor) film. In an image obtained withTEM, a crystal grain boundary cannot be found clearly in the nc-OS filmin some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Note that there isno regularity of crystal orientation between different crystal parts inthe nc-OS film. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the no-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a diffraction pattern like ahalo pattern appears in a selected-area electron diffraction pattern ofthe nc-OS film which is obtained by using an electron beam having aprobe diameter (e.g., larger than or equal to 50 nm) larger than thediameter of a crystal part. Meanwhile, spots are shown in a nanobeamelectron diffraction pattern of the nc-OS film obtained by using anelectron beam having a probe diameter close to, or smaller than thediameter of a crystal part. Further, in a nanobeam electron diffractionpattern of the nc-OS film, regions with high luminance in a circular(ring) pattern are shown in some cases. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS film, a plurality of spots are shown ina ring-like region in some cases (see FIG. 18B).

The nc-OS film is an oxide semiconductor film that has high regularityas compared to an amorphous oxide semiconductor film. Therefore, thenc-OS film has a lower density of defect states than an amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that a film which forms the oxide semiconductor layer may be astacked film including two or more films of an amorphous oxidesemiconductor film, a microcrystalline oxide semiconductor film, and aCAAC-OS film, for example.

In the case where the oxide semiconductor film has a plurality ofstructures, the structures can be analyzed using nanobeam electrondiffraction in some cases.

FIG. 18C illustrates a transmission electron diffraction measurementapparatus which includes an electron gun chamber 210, an optical system212 below the electron gun chamber 210, a sample chamber 214 below theoptical system 212, an optical system 216 below the sample chamber 214,an observation chamber 220 below the optical system 216, a camera 218installed in the observation chamber 220, and a film chamber 222 belowthe observation chamber 220. The camera 218 is provided to face towardthe inside of the observation chamber 220. Note that the film chamber222 is not necessarily provided.

FIG. 18D illustrates an internal structure of the transmission electrondiffraction measurement apparatus illustrated in FIG. 18C. In thetransmission electron diffraction measurement apparatus, a substance 228which is positioned in the sample chamber 214 is irradiated withelectrons emitted from an electron gun installed in the electron gunchamber 210 through the optical system 212. Electrons passing throughthe substance 228 enter a fluorescent plate 232 provided in theobservation chamber 220 through the optical system 216. A pattern whichdepends on the intensity of the incident electrons appears in thefluorescent plate 232, so that the transmitted electron diffractionpattern can be measured.

The camera 218 is installed so as to face the fluorescent plate 232 andcan take a picture of a pattern appearing in the fluorescent plate 232.An angle formed by a straight line which passes through the center of alens of the camera 218 and the center of the fluorescent plate 232 andthe fluorescent plate 232 is, for example, 15° or more and 80° or less,30° or more and 75° or less, or 45° or more and 70° or less. As theangle is reduced, distortion of the transmission electron diffractionpattern taken by the camera 218 becomes larger. Note that if the angleis obtained in advance, the distortion of an obtained transmissionelectron diffraction pattern can be corrected. Note that the camera 218can be provided in the film chamber 222 in some cases. For example, thecamera 218 may be set in the film chamber 222 so as to be opposite tothe incident direction of electrons 224 enter. In this case, atransmission electron diffraction pattern with few distortion can betaken from the rear surface of the fluorescent plate 232.

A holder for fixing the substance 228 that is a sample is provided inthe sample chamber 214. Electrons which passes through the substance 228penetrate the holder. The holder may have, for example, a function ofmoving the substance 228 in the direction of the X, Y, and Z axes. Themovement function of the holder may have an accuracy of moving thesubstance in the range of, for example, 1 nm to 10 nm, 5 nm to 50 nm, 10nm to 100 nm, 50 nm to 500 nm, and 100 nm to 1 μm. The range ispreferably determined to be an optimal range for the structure of thesubstance 228.

Then, a method for measuring a transmission electron diffraction patternof a substance by the transmission electron diffraction measurementapparatus described above will be described.

For example, changes in the structure of a substance can be observed bychanging the irradiation position of the electrons 224 that are ananobeam on the substance (or by scanning) as illustrated in FIG. 18D.At this time, when the substance 228 is a CAAC-OS film, a diffractionpattern shown in FIG. 18A is observed. When the substance 228 is anno-OS film, a diffraction pattern shown in FIG. 18B is observed.

Even when the substance 228 is a CAAC-OS film, a diffraction patternsimilar to that of an nc-OS film or the like is partly observed in somecases. Therefore, whether or not a CAAC-OS film is favorable can bedetermined by the proportion of a region where a diffraction pattern ofa CAAC-OS film is observed in a predetermined area (also referred to asproportion of CAAC). For example, in the case of a favorable CAAC-OSfilm, the proportion of CAAC is 60% or higher, preferably 80% or higher,further preferably 90% or higher, still preferably 95% or higher. Notethat a proportion of a region other than that of the CAAC region isreferred to as the proportion of non-CAAC.

For example, transmission electron diffraction patterns were obtained byscanning a top surface of a sample including a CAAC-OS film obtainedjust after deposition (represented as “as-sputtered”) and a top surfaceof a sample including a CAAC-OS film subjected to heat treatment at 450°C. in an atmosphere containing oxygen. Here, the proportion of CAAC wasobtained in such a manner that diffraction patterns were observed byscanning for 60 seconds at a rate of 5 nm/second and the obtaineddiffraction patterns were converted into still images every 0.5 seconds.Note that as an electron beam, a nanobeam with a probe diameter of 1 nmwas used. The above measurement was performed on six samples. Theproportion of CAAC was calculated using the average value of the sixsamples.

FIG. 19A shows the proportion of CAAC in each sample. The proportion ofCAAC of the CAAC-OS film obtained just after the deposition was 75.7%(the proportion of non-CAAC was 24.3%). The proportion of CAAC of theCAAC-OS film subjected to the heat treatment at 450° C. was 85.3% (theproportion of non-CAAC was 14.7%). These results show that theproportion of CAAC obtained after the heat treatment at 450° C. ishigher than that obtained just after the deposition. That is, heattreatment at a high temperature (e.g., higher than or equal to 400° C.)reduces the proportion of non-CAAC (increases the proportion of CAAC).Further, the above results also indicate that even when the temperatureof the heat treatment is lower than 500° C., the CAAC-OS film can have ahigh proportion of CAAC.

Here, most of diffraction patterns different from that of a CAAC-OS filmare diffraction patterns similar to that of an nc-OS film. Further, anamorphous oxide semiconductor film was not able to be observed in themeasurement region. Therefore, the above results suggest that the regionhaving a structure similar to that of an nc-OS film is rearranged by theheat treatment owing to the influence of the structure of the adjacentregion, whereby the region becomes CAAC.

FIGS. 19B and 19C are planar TEM images of the CAAC-OS film obtainedjust after the deposition and the CAAC-OS film subjected to the heattreatment at 450° C., respectively. Comparison between FIGS. 19B and 19Cshows that the CAAC-OS film subjected to the heat treatment at 450° C.has more uniform film quality. That is, the heat treatment at a hightemperature improves the film quality of the CAAC-OS film.

With such a measurement method, the structure of an oxide semiconductorfilm having a plurality of structures can be analyzed in some cases.

The semiconductor device of one embodiment of the present invention canbe formed using an oxide semiconductor film having any of the abovestructures.

<Deposition Model>

Examples of deposition models of a CAAC-OS film and an nc-OS film aredescribed below.

FIG. 40A is a schematic diagram of a deposition chamber illustrating astate where the CAAC-OS film is formed by a sputtering method.

A target 1130 is attached to a backing plate. Under the target 1130 andthe backing plate, a plurality of magnets are provided. The plurality ofmagnets cause a magnetic field over the target 1130. A sputtering methodin which the disposition speed is increased by utilizing a magneticfield of magnets is referred to as a magnetron sputtering method.

The target 1130 has a polycrystalline structure in which a cleavageplane exists in at least one crystal grain. Note that the details of thecleavage plane are described later.

A substrate 1120 is placed to face the target 1130, and the distance d(also referred to as a target-substrate distance (T-S distance)) isgreater than or equal to 0.01 m and less than or equal to 1 m,preferably greater than or equal to 0.02 m and less than or equal to 0.5m. The deposition chamber is mostly filled with a deposition gas (e.g.,an oxygen gas, an argon gas, or a mixed gas containing oxygen at 50 vol% or higher) and controlled to higher than or equal to 0.01 Pa and lowerthan or equal to 100 Pa, preferably higher than or equal to 0.1 Pa andlower than or equal to 10 Pa. Here, discharge starts by application of avoltage at a certain value or higher to the target 1130, and plasma isobserved. Note that the magnetic field over the target 1130 forms ahigh-density plasma region. In the high-density plasma region, thedeposition gas is ionized, so that anion 1101 is generated. Examples ofthe ion 1101 include an oxygen cation (O⁺) and an argon cation (Ar⁺).

The ion 1101 is accelerated to the target 1130 side by an electricfield, and collides with the target 1130 eventually. At this time, apellet 1100 a and a pellet 1100 b which are flat-plate-like orpellet-like sputtered particles are separated and sputtered from thecleavage plane. Note that structures of the pellet 1100 a and the pellet1100 b may be distorted by an impact of collision of the ion 1101.

The pellet 1100 a is a flat-plate-like or pellet-like sputtered particlehaving a triangle plane, e.g., a regular triangle plane. The pellet 1100b is a flat-plate-like or pellet-like sputtered particle having ahexagon plane, e.g., regular hexagon plane. Note that flat-plate-like orpellet-like sputtered particles such as the pellet 1100 a and the pellet1100 b are collectively called pellets 1100. The shape of a flat planeof the pellet 1100 is not limited to a triangle or a hexagon. Forexample, the flat plane may have a shape formed by combining greaterthan or equal to 2 and less than or equal to 6 triangles. For example, asquare (rhombus) is formed by combining two triangles (regulartriangles) in some cases.

The thickness of the pellet 1100 is determined depending on the kind ofthe deposition gas and the like. The thicknesses of the pellets 1100 arepreferably uniform; the reasons thereof are described later. Inaddition, the sputtered particle preferably has a pellet shape with asmall thickness as compared to a dice shape with a large thickness.

The pellet 1100 receives charge when passing through the plasma, so thatside surfaces of the pellet 1100 are negatively or positively charged insome cases. The pellet 1100 includes an oxygen atom on its side surface,and the oxygen atom may be negatively charged. For example, a case inwhich the pellet 1100 a includes, on its side surfaces, oxygen atomsthat are negatively charged is illustrated in FIG. 42. As in this view,when the side surfaces are charged in the same polarity, charges repeleach other, and accordingly, the pellet can maintain a flat-plate shape.In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibilitythat an oxygen atom bonded to an indium atom is negatively charged.There is another possibility that an oxygen atom bonded to an indiumatom, a gallium atom, and a zinc atom is negatively charged.

As shown in FIG. 40A, the pellet 1100 flies like a kite in plasma andflutters up to the substrate 1120. Since the pellets 1100 are charged,when the pellet 1100 gets close to a region where another pellet 1100has already been deposited, repulsion is generated. Here, above thesubstrate 1120, a magnetic field is generated in a direction parallel toa top surface of the substrate 1120. A potential difference is givenbetween the substrate 1120 and the target 1130, and accordingly, currentflows from the substrate 1120 toward the target 1130. Thus, the pellet1100 is given a force (Lorentz force) on the top surface of thesubstrate 1120 by an effect of the magnetic field and the current (seeFIG. 43). This is explainable with Fleming's left-hand rule. In order toincrease a force applied to the pellet 1100, it is preferable toprovide, on the top surface, a region where the magnetic field in adirection parallel to the top surface of the substrate 1120 is 10 G orhigher, preferably 20 G or higher, further preferably 30 G or higher,still further preferably 50 G or higher. Alternatively, it is preferableto provide, on the top surface, a region where the magnetic field in adirection parallel to the top surface of the substrate is 1.5 times orhigher, preferably twice or higher, further preferably 3 times orhigher, still further preferably 5 times or higher as high as themagnetic field in a direction perpendicular to the top surface of thesubstrate 1120.

Furthermore, the substrate 1120 is heated, and resistance such asfriction between the pellet 1100 and the substrate 1120 is low. As aresult, as illustrated in FIG. 44A, the pellet 1100 glides above thesurface of the substrate 1120. The glide of the pellet 1100 is caused ina state where the flat plane faces the substrate 1120. Then, asillustrated in FIG. 44B, when the pellet 1100 reaches the side surfaceof another pellet 1100 that has been already deposited, the sidesurfaces of the pellets 1100 are bonded. At this time, the oxygen atomon the side surface of the pellet 1100 is released. With the releasedoxygen atom, oxygen vacancies in a CAAC-OS are filled in some cases;thus, the CAAC-OS has a low density of defect states.

Further, the pellet 1100 is heated on the substrate 1120, whereby atomsare rearranged, and the structure distortion caused by the collision ofthe ion 1101 can be reduced. The pellet 1100 whose structure distortionis reduced is substantially single crystal. Even when the pellets 1100are heated after being bonded, expansion and contraction of the pellet1100 itself hardly occur, which is caused by turning the pellet 1100into substantially single crystal. Thus, formation of defects such as agrain boundary due to expansion of a space between the pellets 1100 canbe prevented, and accordingly, generation of crevasses can be prevented.Further, the space is filled with elastic metal atoms and the like,whereby the elastic metal atoms have a function, like a highway, ofjointing side surfaces of the pellets 1100 which are not aligned witheach other.

It is considered that as shown in such a model, the pellets 1100 aredeposited over the substrate 1120. Thus, a CAAC-OS film can be depositedeven when a surface over which a film is formed (film formation surface)does not have a crystal structure, which is different from filmdeposition by epitaxial growth. For example, even when a surface (filmformation surface) of the substrate 1120 has an amorphous structure, aCAAC-OS film can be formed.

Further, it is found that in formation of the CAAC-OS, the pellets 1100are arranged in accordance with a surface shape of the substrate 1120that is the film formation surface even when the film formation surfacehas unevenness besides a flat surface. For example, in the case wherethe surface of the substrate 1120 is flat at the atomic level, thepellets 1100 are arranged so that flat planes parallel to the a-b planeface downwards; thus, a layer with a uniform thickness, flatness, andhigh crystallinity is formed. By stacking n layers (n is a naturalnumber), the CAAC-OS can be obtained (see FIG. 40B).

In the case where the top surface of the substrate 1120 has unevenness,a CAAC-OS where n layers (n is a natural number) in each of which thepellets 1100 are arranged along a convex surface are stacked is formed.Since the substrate 1120 has unevenness, a gap is easily generatedbetween in the pellets 1100 in the CAAC-OS in some cases. Note thatowing to intermolecular force, the pellets 1100 are arranged so that agap between the pellets is as small as possible even on the unevennesssurface. Therefore, even when the formation surface has unevenness, aCAAC-OS with high crystallinity can be formed (see FIG. 40C).

As a result, laser crystallization is not needed for formation of aCAAC-OS, and a uniform film can be formed even over a large-sized glasssubstrate.

Since the CAAC-OS film is deposited in accordance with such a model, thesputtered particle preferably has a pellet shape with a small thickness.Note that in the case where the sputtered particle has a dice shape witha large thickness, planes facing the substrate 1120 are not uniform andthus, the thickness and the orientation of the crystals cannot beuniform in some cases.

According to the deposition model described above, a CAAC-OS with highcrystallinity can be formed even on a film formation surface with anamorphous structure.

Further, formation of a CAAC-OS can be described with a deposition modelincluding a zinc oxide particle besides the pellet 1100.

The zinc oxide particle reaches the substrate 1120 before the pellet1100 does because the zinc oxide particle is smaller than the pellet1100 in mass. On the surface of the substrate 1120, crystal growth ofthe zinc oxide particle preferentially occurs in the horizontaldirection, so that a thin zinc oxide layer is formed. The zinc oxidelayer has c-axis alignment. Note that c-axes of crystals in the zincoxide layer are aligned in the direction parallel to a normal vector ofthe substrate 1120. The zinc oxide layer serves as a seed layer thatmakes a CAAC-OS grow and thus has a function of increasing crystallinityof the CAAC-OS. The thickness of the zinc oxide layer is greater than orequal to 0.1 nm and less than or equal to 5 nm, mostly greater than orequal to 1 nm and less than or equal to 3 nm. Since the zinc oxide layeris sufficiently thin, a grain boundary is hardly observed.

Thus, in order to deposit a CAAC-OS with high crystallinity, a targetcontaining zinc at a proportion higher than that of the stoichiometriccomposition is preferably used.

An nc-OS can be understood with a deposition model illustrated in FIG.41. Note that a difference between FIG. 41 and FIG. 40A lies only in thefact that whether the substrate 1120 is heated or not.

Thus, the substrate 1120 is not heated, and a resistance such asfriction between the pellet 1100 and the substrate 1120 is high. As aresult, the pellets 1100 cannot 30 glide on the surface of the substrate1120 and are stacked randomly, thereby forming an nc-OS.

<Cleavage Plane>

A cleavage plane that has been mentioned in the deposition model of theCAAC-OS will be described below.

First, a cleavage plane of the target is described with reference toFIGS. 45A and 45B. FIGS. 45A and 45B show the crystal structure ofInGaZnO₄. Note that FIG. 45A shows the structure of the case where anInGaZnO₄ crystal is observed from a direction parallel to the b-axiswhen the c-axis is in an upward direction. Furthermore, FIG. 45B showsthe structure of the case where the InGaZnO₄ crystal is observed from adirection parallel to the c-axis.

Energy needed for cleavage at each of crystal planes of the InGaZnO₄crystal is calculated by the first principles calculation. Note that a“pseudopotential” and density functional theory program (CASTEP) usingthe plane wave basis are used for the calculation. Note that anultrasoft type pseudopotential is used as the pseudopotential. Further,GGA/PBE is used as the functional. Cut-off energy is 400 eV.

Energy of a structure in an initial state is obtained after structuraloptimization including a cell size is performed. Further, energy of astructure after the cleavage at each plane is obtained after structuraloptimization of atomic arrangement is performed in a state where thecell size is fixed.

On the basis of the structure of the InGaZnO₄ crystal in FIGS. 45A and45B, a structure cleaved at any one of a first plane, a second plane, athird plane, and a fourth plane is formed and subjected to structuraloptimization calculation in which the cell size is fixed. Here, thefirst plane is a crystal plane between a Ga—Zn—O layer and an In—O layerand is parallel to the (001) plane (or the a-b plane) (see FIG. 45A).The second plane is a crystal plane between a Ga—Zn—O layer and aGa—Zn—O layer and is parallel to the (001) plane (or the a-b plane) (seeFIG. 45A). The third plane is a crystal plane parallel to the (110)plane (see FIG. 45B). The fourth plane is a crystal plane parallel tothe (100) plane (or the b-c plane) (see FIG. 45B).

Under the above conditions, the energy of the structure at each planeafter the cleavage is calculated. Next, a difference between the energyof the structure after the cleavage and the energy of the structure inthe initial state is divided by the area of the cleavage plane; thus,cleavage energy which serves as a measure of easiness of cleavage ateach plane is calculated. Note that the energy of a structure indicatesenergy obtained in such a manner that electronic kinetic energy ofelectrons included in the structure and interactions between atomsincluded in the structure, between the atom and the electron, andbetween the electrons are considered.

As calculation results, the cleavage energy of the first plane was 2.60J/m², that of the second plane was 0.68 J/m², that of the third planewas 2.18 J/m², and that of the fourth plane was 2.12 J/m² (see Table 1).

TABLE 1 Cleavage energy [J/m²] First plane 2.60 Second plane 0.68 Thirdplane 2.18 Fourth plane 2.12

From the calculations, in the structure of the InGaZnO₄ crystal in FIGS.45A and 45B, the cleavage energy of the second plane is the lowest. Inother words, a plane between a Ga—Zn—O layer and a Ga—Zn—O layer iscleaved most easily (cleavage plane). Therefore, in this specification,the cleavage plane indicates the second plane, which is a plane wherecleavage is performed most easily.

Since the cleavage plane is the second plane between the Ga—Zn—O layerand the Ga—Zn—O layer, the InGaZnO₄ crystals in FIG. 45A can beseparated at a plane equivalent to two second planes. Therefore, in thecase where an ion or the like is made to collide with a target, awafer-like unit (we call this a pellet) which is cleaved at a plane withthe lowest cleavage energy is thought to be blasted off as the minimumunit. In that case, a pellet of InGaZnO₄ includes three layers: aGa—Zn—O layer, an In—O layer, and a Ga—Zn—O layer.

The cleavage energies of the third plane (crystal plane parallel to the(110) plane) and the fourth plane (crystal plane parallel to the (100)plane (or the b-c plane)) are lower than that of the first plane(crystal plane between the Ga—Zn—O layer and the In—O layer and planethat is parallel to the (001) plane (or the a-b plane)), which suggeststhat most of the flat planes of the pellets have triangle shapes orhexagonal shapes.

Next, through classical molecular dynamics calculation, on theassumption of an InGaZnO₄ crystal having a homologous structure as atarget, a cleavage plane in the case where the target is sputtered usingargon (Ar) or oxygen (O) is examined. FIG. 46A shows a cross-sectionalstructure of an InGaZnO₄ crystal (2688 atoms) used for the calculation,and FIG. 46B shows a top structure thereof. Note that a fixed layer inFIG. 46A prevents the positions of the atoms from moving. A temperaturecontrol layer in FIG. 46A is a layer whose temperature is constantly setto fixed temperature (300 K).

For the classical molecular dynamics calculation, Materials Explorer 5.0manufactured by Fujitsu Limited. is used. Note that the initialtemperature, the cell size, the time step size, and the number of stepsare set to be 300 K, a certain size, 0.01 fs, and ten million,respectively. In calculation, an atom to which an energy of 300 eV isapplied is made to enter a cell from a direction perpendicular to thea-b plane of the InGaZnO₄ crystal under the conditions.

FIG. 47A shows atomic order when 99.9 picoseconds have passed afterargon enters the cell including the InGaZnO₄ crystal in FIGS. 46A and46B. FIG. 47B shows atomic order when 99.9 picoseconds have passed afteroxygen enters the cell. Note that in FIGS. 47A and 47B, part of thefixed layer in FIG. 46A is omitted.

According to FIG. 47A, in a period from entry of argon into the cell towhen 99.9 picoseconds have passed, a crack is formed from the cleavageplane corresponding to the second plane in FIG. 45A. Thus, in the casewhere argon collides with the InGaZnO₄ crystal and the uppermost surfaceis the second plane (the zero-th), a large crack is found to be formedin the second plane (the second).

On the other hand, according to FIG. 47B, in a period from entry ofoxygen into the cell to when 99.9 picoseconds have passed, a crack isfound to be formed from the cleavage plane corresponding to the secondplane in FIG. 45A. Note that in the case where oxygen collides with thecell, a large crack is found to be formed in the second plane (thefirst) of the InGaZnO₄ crystal.

Accordingly, it is found that an atom (ion) collides with a targetincluding an InGaZnO₄ crystal having a homologous structure from theupper surface of the target, the InGaZnO₄ crystal is cleaved along thesecond plane, and a flat-plate-like sputtered particle (pellet) isseparated. It is also found that the pellet formed in the case whereoxygen collides with the cell is smaller than that formed in the casewhere argon collides with the cell.

The above calculation suggests that the separated pellet includes adamaged region. In some cases, the damaged region included in the pelletcan be repaired in such a manner that a defect caused by the damagereacts with oxygen.

Here, difference in size of the pellet depending on atoms which are madeto collide is studied.

FIG. 48A shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after argon enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48A corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

According to FIG. 48A, when argon collides with gallium (Ga) of thefirst layer (Ga—Zn—O layer), gallium collides with zinc (Zn) of thethird layer (Ga—Zn—O layer) and then, zinc reaches the vicinity of thesixth layer (Ga—Zn—O layer). Note that the argon which collides with thegallium is sputtered to the outside. Accordingly, in the case whereargon collides with the target including the InGaZnO₄ crystal, a crackis thought to be formed in the second plane (the second) in FIG. 46A.

FIG. 48B shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after oxygen enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48B corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

On the other hand, according to FIG. 48B, when oxygen collides withgallium (Ga) of the first layer (Ga—Zn—O layer), gallium collides withzinc (Zn) of the third layer (Ga—Zn—O layer) and then, zinc does notreach the fifth layer (In—O layer). Note that the oxygen which collideswith the gallium is sputtered to the outside. Accordingly, in the casewhere oxygen collides with the target including the InGaZnO₄ crystal, acrack is thought to be formed in the second plane (the first) in FIG.46A.

This calculation also shows that the InGaZnO₄ crystal with which an atom(ion) collides is separated from the cleavage plane.

In addition, a difference in depth of a crack is examined in view ofconservation laws. The energy conservation law and the law ofconservation of momentum can be represented by the following formula (1)and the following formula (2). Here, E represents energy of argon oroxygen before collision (300 eV), m_(A) represents mass of argon oroxygen, v_(A) represents the speed of argon or oxygen before collision,v′_(A) represents the speed of argon or oxygen after collision, m_(Ga)represents mass of gallium, v_(Ga) represents the speed of galliumbefore collision, and v′_(Ga) represents the speed of gallium aftercollision.

[Formula  1] $\begin{matrix}{E = {{\frac{1}{2}m_{A}v_{A}^{2}} + {\frac{1}{2}m_{Ga}v_{Ga}^{2}}}} & (1)\end{matrix}$[Formula 2]m _(A) v _(A) +m _(Ga) v _(Ga) =m _(A) v′ _(A) +m _(Ga) v′ _(Ga)  (2)

On the assumption that collision of argon or oxygen is elasticcollision, the relationship among v_(A), v′_(A), v_(Ga), and v′_(Ga) canbe represented by the following formula (3).[Formula 3]v′ _(A) −v′ _(Ga)=−(v _(A) −v _(Ga))  (3)

From the formulae (1), (2), and (3), on the assumption that v_(Ga) is 0,the speed of gallium v′_(Ga) after collision of argon or oxygen can berepresented by the following formula (4).

[Formula  4] $\begin{matrix}{v_{Ga}^{\prime} = {{\frac{\sqrt{m_{A}}}{m_{A} + m_{Ga}} \cdot 2}\sqrt{2E}}} & (4)\end{matrix}$

In the formula (4), mass of argon or oxygen is substituted into m_(A),whereby the speeds after collision of the atoms are compared. In thecase where the argon and the oxygen have the same energy beforecollision, the speed of gallium in the case where argon collides withthe gallium was found to be 1.24 times as high as that in the case whereoxygen collides with the gallium. Thus, the energy of the gallium in thecase where argon collides with the gallium is higher than that in thecase where oxygen collides with the gallium by the square of the speed.

The speed (energy) of gallium after collision in the case where argoncollides with the gallium is found to be higher than that in the casewhere oxygen collides with the gallium. Accordingly, it is consideredthat a crack is formed at a deeper position in the case where argoncollides with the gallium than in the case where oxygen collides withthe gallium.

The above calculation shows that when sputtering is performed using atarget including the InGaZnO₄ crystal having a homologous structure,separation occurs from the cleavage plane to form a pellet. On the otherhand, even when sputtering is performed on a region having anotherstructure of a target without the cleavage plane, a pellet is notformed, and a sputtered particle with an atomic-level size which isminuter than a pellet is formed. Because the sputtered particle issmaller than the pellet, the sputtered particle is thought to be removedthrough a vacuum pump connected to a sputtering apparatus. Therefore, amodel in which particles with a variety of sizes and shapes fly to asubstrate and are deposited hardly applies to the case where sputteringis performed using a target including the InGaZnO₄ crystal having ahomologous structure. The model illustrated in FIG. 40A where sputteredpellets are deposited to form a CAAC-OS is a reasonable model.

The CAAC-OS deposited in such a manner has a density substantially equalto that of a single crystal OS. For example, the density of the singlecrystal OS film having a homologous structure of InGaZnO₄ is 6.36 g/cm³,and the density of the CAAC-OS film having substantially the same atomicratio is approximately 6.3 g/cm³.

FIGS. 49A and 49B show atomic order of cross sections of an In—Ga—Znoxide (see FIG. 49A) that is a CAAC-OS deposited by sputtering and atarget thereof (see FIG. 49B). For observation of atomic arrangement, ahigh-angle annular dark field scanning transmission electron microscopy(HAADF-STEM) is used. In the case of observation by HAADF-STEM, theintensity of an image of each atom is proportional to the square of itsatomic number. Therefore, Zn (atomic number: 30) and Ga (atomic number:31), whose atomic numbers are close to each other, are hardlydistinguished from each other. A Hitachi scanning transmission electronmicroscope HD-2700 is used for the HAADF-STEM.

When FIG. 49A and FIG. 49B are compared, it is found that the CAAC-OSand the target each have a homologous structure and atomic order in theCAAC-OS correspond to that in the target. Thus, as illustrated in thedeposition model in FIG. 40A, the crystal structure of the target istransferred, whereby a CAAC-OS is formed.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, an oxygen vacancy of an oxide semiconductor film isdescribed in detail below.

<(1) Ease of Formation and Stability of V_(o)H>

In the case where an oxide semiconductor film (hereinafter referred toas IGZO) is a complete crystal, H preferentially diffuses along the a-bplane at a room temperature. In heat treatment at 450° C., H diffusesalong the a-b plane and in the c-axis direction. Here, description ismade on whether H easily enters an oxygen vacancy V_(o) if the oxygenvacancy V_(o) exists in IGZO. A state in which H is in an oxygen vacancyV_(o) is referred to as V_(o)H.

An InGaZnO₄ crystal model shown in FIG. 20 was used for calculation. Theactivation barrier (E_(a)) along the reaction path where H in V_(o)H isreleased from V_(o) and bonded to oxygen was calculated by a nudgedelastic band (NEB) method. The calculation conditions are shown in Table2.

TABLE 2 Software VASP Calculation method NEB method Functional GGA-PBEPseudopotential PAW Cut-off energy 500 eV K points 2 × 2 × 3

In the InGaZnO₄ crystal model, there are oxygen sites 1 to 4 as shown inFIG. 20 which differ from each other in metal elements bonded to oxygenand the number of bonded metal elements. Here, calculation was made onthe oxygen sites 1 and 2 in which an oxygen vacancy V_(o) is easilyformed.

First, calculation was made on the oxygen site in which an oxygenvacancy V_(o) is easily formed: an oxygen site 1 that was bonded tothree In atoms and one Zn atom.

FIG. 21A shows a model in the initial state and FIG. 21B shows a modelin the final state. FIG. 22 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in an oxygen vacancyV_(o) (V_(o)H), and the final state refers to a structure including anoxygen vacancy V_(o) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in an oxygen vacancy V_(o) toanother oxygen atom needs an energy of approximately 1.52 eV, whileentry of H bonded to O into an oxygen vacancy V_(o) needs an energy ofapproximately 0.46 eV.

Reaction frequency (Γ) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and Formula 5. In Formula5, k_(B) represents the Boltzmann constant and T represents the absolutetemperature.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack & \; \\{\Gamma = {v\mspace{11mu}{\exp\left( {- \frac{E_{a}}{k_{B}T}} \right)}}} & \;\end{matrix}$

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor v=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 21A to the model shown in FIG. 21B was 5.52×10⁰[1/sec], whereas the frequency of H transfer from the model shown inFIG. 21B to the model shown in FIG. 21A was 1.82×10⁹ [1/sec]. Thissuggests that H diffusing in IGZO is likely to form V_(o)H if an oxygenvacancy V_(o) exists in the neighborhood, and H is unlikely to bereleased from the oxygen vacancy V_(o) once V_(o)H is formed.

Next, calculation was made on the oxygen site in which an oxygen vacancyV_(o) is easily formed: an oxygen site 2 that was bonded to one Ga atomand two Zn atoms.

FIG. 23A shows a model in the initial state and FIG. 23B shows a modelin the final state. FIG. 24 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in an oxygen vacancyV_(o) (V_(o)H), and the final state refers to a structure including anoxygen vacancy V_(o) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in an oxygen vacancy V_(o) toanother oxygen atom needs an energy of approximately 1.75 eV, whileentry of H bonded to O in an oxygen vacancy V_(o) needs an energy ofapproximately 0.35 eV.

Reaction frequency (Γ) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and Formula 5.

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor v=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 23A to the model shown in FIG. 23B was 7.53×10⁻²[1/sec], whereas the frequency of H transfer from the model shown inFIG. 23B to the model shown in FIG. 23A was 1.44×10¹⁰ [1/sec]. Thissuggests that H is unlikely to be released from the oxygen vacancy V_(o)once V_(o)H is formed.

From the above results, it was found that H in IGZO easily diffused inannealing and if an oxygen vacancy V_(o) existed, H was likely to enterthe oxygen vacancy V_(o) to be V_(o)H.

<(2) Transition Level of V-H>

The calculation by the NEB method, which was described in <(1) Ease offormation and stability of V_(o)H>, indicates that in the case where anoxygen vacancy V_(o) and H exist in IGZO, the oxygen vacancy V_(o) and Heasily form V_(o)H and V_(o)H is stable. To determine whether V_(o)H isrelated to a carrier trap, the transition level of V_(o)H wascalculated.

The model used for calculation is an InGaZnO₄ crystal model (112 atoms).V_(o)H models of the oxygen sites 1 and 2 shown in FIG. 20 were made tocalculate the transition levels. The calculation conditions are shown inTable 3.

TABLE 3 Software VASP Model InGaZnO₄ crystal model (112 atoms)Functional HSE06 Mixture ratio of exchange terms 0.25 PseudopotentialGGA-PBE Cut-off energy 800 eV K points 1 × 1 × 1

The mixture ratio of exchange terms was adjusted to have a band gapclose to the experimental value. As a result, the band gap of theInGaZnO₄ crystal model without defects was 3.08 eV that is close to theexperimental value, 3.15 eV.

The transition level (ε(q/q′)) of a model having defect D can becalculated by the following Formula 6. Note that ΔE(D^(q)) representsthe formation energy of defect D at charge q, which is calculated byFormula 7.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack & \; \\{{ɛ\left( {q/q^{\prime}} \right)} = \frac{{\Delta\;{E\left( D^{q} \right)}} - {\Delta\;{E\left( D^{q^{\prime}} \right)}}}{q^{\prime} - q}} & \;\end{matrix}$

$\begin{matrix}{\mspace{79mu}\left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack} & \; \\{{\Delta\;{E\left( D^{q} \right)}} = {{E_{tot}\left( D^{q} \right)} - {E_{tot}({bulk})} + {\sum\limits_{i}^{\;}\;{\Delta\; n_{i}\mu_{i}}} + {q\left( {ɛ_{VBM} + {\Delta\; V_{q}} + E_{F}} \right)}}} & \;\end{matrix}$

In Formulae 6 and 7, E_(tot)(D^(q)) represents the total energy of themodel having defect D at the charge q, E_(tot)(bulk) represents thetotal energy in a model without defects (complete crystal), Δn_(i)represents a change in the number of atoms i contributing to defects,μ_(i) represents the chemical potential of atom i, ε_(VBM) representsthe energy of the valence band maximum in the model without defects,ΔV_(q) represents the correction term relating to the electrostaticpotential, and E_(F) represents the Fermi energy.

FIG. 25 shows the transition levels of V_(o)H obtained from the aboveformulae. The numbers in FIG. 25 represent the depth from the conductionband minimum. In FIG. 25, the transition level of V_(o)H in the oxygensite 1 is at 0.05 eV from the conduction band minimum, and thetransition level of V_(o)H in the oxygen site 2 is at 0.11 eV from theconduction band minimum. Therefore, these V_(o)H would be related toelectron traps, that is, V_(o)H was found to behave as a donor. It wasalso found that IGZO including V_(o)H had conductivity.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, an example of a display device that includes any ofthe transistors and the capacitors described in the embodiment above isdescribed below with reference to FIG. 26, FIG. 27, and FIG. 28.

FIG. 26 is a top view of an example of a display device. A displaydevice 700 illustrated in FIG. 26 includes a pixel portion 702 providedover a first substrate 701; a source driver circuit portion 704 and agate driver circuit portion 706 provided over the first substrate 701; asealant 712 provided to surround the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706; anda second substrate 705 provided to face the first substrate 701. Thefirst substrate 701 and the second substrate 705 are sealed with thesealant 712. That is, the pixel portion 702, the source driver circuitportion 704, and the gate driver circuit portion 706 are sealed with thefirst substrate 701, the sealant 712, and the second substrate 705.Although not illustrated in FIG. 26, a display element is providedbetween the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 electrically connected to the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706 isprovided in a region different from the region which is surrounded bythe sealant 712 and positioned over the first substrate 701.Furthermore, an FPC 716 is connected to the FPC terminal portion 708,and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706 through the FPC 716. Furthermore, a signal line 710 isconnected to the pixel portion 702, the source driver circuit portion704, the gate driver circuit portion 706, and the FPC terminal portion708. Various signals and the like are applied to the pixel portion 702,the source driver circuit portion 704, the gate driver circuit portion706, and the FPC terminal portion 708 via the signal line 710 from theFPC 716.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. An example of the display device 700 in which thesource driver circuit portion 704 and the gate driver circuit portion706 are formed over the first substrate 701 where the pixel portion 702is also formed is described; however, the structure is not limitedthereto. For example, only the gate driver circuit portion 706 may beformed over the first substrate 701 or only the source driver circuitportion 704 may be formed over the first substrate 701. In this case, asubstrate where a source driver circuit, a gate driver circuit, or thelike is formed (e.g., a driver-circuit substrate formed using asingle-crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted on the first substrate 701. Note that there is noparticular limitation on the method of connecting a separately prepareddriver circuit substrate, and a chip on glass (COG) method, a wirebonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors. As the plurality of transistors, anyof the transistors that are the semiconductor devices of embodiments ofthe present invention can be used. In the pixel portion 702, any of thetransistors and capacitors that are the semiconductor devices ofembodiments of the present invention can be used.

The display device 700 can include any of a variety of elements.Examples of the element include a liquid crystal element, anelectroluminescence (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, electronic ink, an electrophoretic element, agrating light valve (GLV), a plasma display panel (PDP), a displayelement using micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, and a display element including a carbon nanotube. Other thanthe above, display media whose contrast, luminance, reflectivity,transmittance, or the like is changed by electrical or magnetic effectmay be included. Note that examples of display devices having ELelements include an EL display. Examples of display devices includingelectron emitters include a field emission display (FED) and an SED-typeflat panel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed tocontain aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

As a display method in the display device 700, a progressive method, aninterlace method, or the like can be employed. Furthermore, colorelements controlled in a pixel at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of the R pixel,the G pixel, the B pixel, and a W (white) pixel may be included.Alternatively, a color element may be composed of two colors among R, G,and B as in PenTile layout. The two colors may differ among colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Further, the size of a display regionmay be different depending on respective dots of the color components.Embodiments of the disclosed invention are not limited to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used inorder to obtain a full-color display device in which white light (W) fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp) is used. As the coloring layer, red (R),green (G), blue (B), yellow (Y), or the like may be combined asappropriate, for example. With the use of the coloring layer, highercolor reproducibility can be obtained than in the case without thecoloring layer. In this case, by providing a region with the coloringlayer and a region without the coloring layer, white light in the regionwithout the coloring layer may be directly utilized for display. Bypartly providing the region without the coloring layer, a decrease inluminance due to the coloring layer can be suppressed, and 20% to 30% ofpower consumption can be reduced in some cases when an image isdisplayed brightly. Note that in the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light of their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption can be further reduced as compared to the case of using thecoloring layer in some cases.

In this embodiment, a structure including a liquid crystal element andan EL element as display elements is described with reference to FIG. 27and FIG. 28. Note that FIG. 27 is a cross-sectional view along thedashed-dotted line Q-R shown in FIG. 26 and shows a structure includinga liquid crystal element as a display element, whereas FIG. 28 is across-sectional view along the dashed-dotted line Q-R shown in FIG. 26and shows a structure including an EL element as a display element.

Common portions between FIG. 27 and FIG. 28 are described first, andthen different portions are described.

<Common Portions in Display Devices>

The display device 700 illustrated in FIG. 27 and FIG. 28 includes alead wiring portion 711, the pixel portion 702, the source drivercircuit portion 704, and the FPC terminal portion 708. Note that thelead wiring portion 711 includes a signal line 710. The pixel portion702 includes a transistor 750 and a capacitor 790. The source drivercircuit portion 704 includes a transistor 752.

The transistor 750 and the transistor 752 each have a structure similarto that of the transistor 100A described above. Note that the transistor750 and the transistor 752 may each have a structure of the othertransistors described in any of the above embodiments.

The transistors used in this embodiment each include an oxidesemiconductor film which is highly purified and in which formation ofoxygen vacancies is suppressed. In the transistor, the current in an offstate (off-state current) can be made small. Accordingly, an electricalsignal such as an image signal can be held for a longer period, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high speed operation.For example, with such a transistor which can operate at high speed usedfor a liquid crystal display device, a switching transistor in a pixelportion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, the transistor which can operate at highspeed can be used also in the pixel portion, whereby a high-qualityimage can be provided.

The capacitor 790 has a structure similar to that of the capacitor 150Adescribed above.

In FIG. 27 and FIG. 28, an insulating film 766 and a planarizationinsulating film 770 are provided over the transistor 750, the transistor752, and the capacitor 790.

The insulating film 766 can be formed using materials and methodssimilar to that of the insulating film 128 described in the aboveembodiment. The planarization insulating film 770 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film770 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 770 may be employed.

The signal line 710 is formed in the same process as conductive filmsfunctioning as a source electrode and a drain electrode of thetransistor 750 or 752. Note that the signal line 710 may be formed usinga conductive film which is formed in a different process from as asource electrode and a drain electrode of the transistor 750 or 752,e.g., a conductive film functioning as a first gate electrode or aconductive film functioning as a second gate electrode may be used. Inthe case where the signal line 710 is formed using a material containinga copper element, signal delay or the like due to wiring resistance isreduced, which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed in the same process as conductivefilms functioning as a source electrode and a drain electrode of thetransistor 750 or 752. The connection electrode 760 is electricallyconnected to a terminal included in the FPC 716 through the anisotropicconductive film 780.

For example, a glass substrate can be used as the first substrate 701and the second substrate 705. A flexible substrate may be used as thefirst substrate 701 and the second substrate 705. Examples of theflexible substrate include a plastic substrate.

A structure 778 is provided between the first substrate 701 and thesecond substrate 705. The structure 778 is a columnar spacer obtained byselective etching of an insulating film and is provided to control thethickness (cell gap) between the first substrate 701 and the secondsubstrate 705. Alternatively, a spherical spacer may be used as thestructure 778.

Furthermore, a light-blocking film 738 functioning as a black matrix, acoloring film 736 functioning as a color filter, and an insulating film734 in contact with the light-blocking film 738 and the coloring film736 are provided on the second substrate 705 side.

<Structure Example of Display Device Using Liquid Crystal Element asDisplay Element>

The display device 700 illustrated in FIG. 27 includes a liquid crystalelement 775. The liquid crystal element 775 includes a conductive film772, a conductive film 774, and a liquid crystal layer 776. Theconductive film 774 is provided on the second substrate 705 side andfunctions as a counter electrode. The display device 700 in FIG. 27 iscapable of displaying an image in such a manner that transmission ornon-transmission is controlled by change in the alignment state of theliquid crystal layer 776 depending on a voltage applied to theconductive film 772 and the conductive film 774.

The conductive film 772 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 772 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. The conductive film 772 has a function of a reflectiveelectrode. The display device 700 in FIG. 27 is what is called areflective color liquid crystal display device in which external lightis reflected by the conductive film 772 to display an image through thecoloring film 736.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used for the conductive film 772. Forexample, a material including one kind selected from indium (In), zinc(Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material including aluminum orsilver may be used for the conductive film that reflects visible light.In this embodiment, the conductive film that reflects visible light isused for the conductive film 772.

Note that projections and depressions are provided in part of theplanarization insulating film 770 of the pixel portion 702 in thedisplay device 700 in FIG. 27. The projections and depressions can beformed in such a manner that the planarization insulating film 770 isformed using an organic resin film or the like, and projections anddepressions are formed on the surface of the organic resin film. Theconductive film 772 functioning as a reflective electrode is formedalong the projections and depressions. Therefore, when external light isincident on the conductive film 772, the light is reflected diffusely atthe surface of the conductive film 772, whereby visibility can beimproved.

Note that the display device 700 illustrated in FIG. 27 is a reflectivecolor liquid crystal display device given as an example, but a displaytype is not limited thereto. For example, a transmissive color liquidcrystal display device in which the conductive film 772 is a conductivefilm that transmits visible light may be used. In the case of atransmissive color liquid crystal display device, projections anddepressions are not necessarily provided on the planarization insulatingfilm 770.

Although not illustrated in FIG. 27, an alignment film may be providedon a side of the conductive film 772 in contact with the liquid crystallayer 776 and on a side of the conductive film 774 in contact with theliquid crystal layer 776. Although not illustrated in FIG. 27, anoptical member (an optical substrate) and the like such as a polarizingmember, a retardation member, or an anti-reflection member may beprovided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a sidelight, or the like may be used as a lightsource.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a short response time. The liquid crystalcomposition which includes a liquid crystal exhibiting a blue phase anda chiral material is preferable because it has optical isotropy, whichmakes the alignment process unneeded, and has a small viewing angledependence. An alignment film does not need to be provided and rubbingtreatment is thus not necessary; accordingly, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device in the manufacturing processcan be reduced.

In the case where a liquid crystal element is used as the displayelement, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode,a fringe field switching (FFS) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Further, a normally black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may also be used. There are some examples of avertical alignment mode; for example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be employed.

<Display Device Using Light-Emitting Element as Display Element>

The display device 700 illustrated in FIG. 28 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film784, an EL layer 786, and a conductive film 788. The display device 700shown in FIG. 28 is capable of displaying an image by light emissionfrom the EL layer 786 included in the light-emitting element 782.

The conductive film 784 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 784 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. A conductive film which transmits visible light or aconductive film which reflects visible light can be used for theconductive film 784. The conductive film which transmits visible lightcan be formed using a material including one kind selected from indium(In), zinc (Zn), and tin (Sn), for example. The conductive film whichreflects visible light can be formed using a material including aluminumor silver, for example.

In the display device 700 shown in FIG. 28, an insulating film 730 isprovided over the planarization insulating film 770 and the conductivefilm 784. The insulating film 730 covers part of the conductive film784. Note that the light-emitting element 782 has a top emissionstructure. Therefore, the conductive film 788 has a light-transmittingproperty and transmits light emitted from the EL layer 786. Although thetop-emission structure is described as an example in this embodiment,one embodiment of the present invention is not limited thereto. Abottom-emission structure in which light is emitted to the conductivefilm 784 side, or a dual-emission structure in which light is emitted toboth the conductive film 784 side and the conductive film 788 side maybe employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided to overlap withthe insulating film 730 and to be included in the lead wiring portion711 and in the source driver circuit portion 704. The coloring film 736and the light-blocking film 738 are covered with the insulating film734. A space between the light-emitting element 782 and the insulatingfilm 734 is filled with a sealing film 732. Although a structure withthe coloring film 736 is described as the display device 700 shown inFIG. 28, the structure is not limited thereto. In the case where the ELlayer 786 is formed by a separate coloring method, the coloring film 736is not necessarily provided.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, one embodiment of a light-emitting device using thesemiconductor device of one embodiment of the present invention isdescribed. Note that in this embodiment, a structure of a pixel portionof a light-emitting device is described with reference to FIGS. 29A and29B.

In FIG. 29A, a plurality of FETs 500 is formed over a first substrate502, and each of the FETs 500 is electrically connected to alight-emitting element (504R, 504G, 504B, or 504W). Specifically, theFET 500 is electrically connected to a first conductive film 506included in the light-emitting element. Note that the light-emittingelements (504R, 504G, 504B, and 504W) each include the first conductivefilm 506, a second conductive film 507, an EL layer 510, and a thirdconductive film 512.

Furthermore, coloring layers (514R, 514G, 514B, and 514W) are providedin positions facing the corresponding light-emitting elements (504R,504G, 504B, and 504W). Note that the coloring layers (514R, 514G, 514B,and 514W) are provided in contact with a second substrate 516.Furthermore, a sealing film 518 is provided between the first substrate502 and the second substrate 516. For example, a glass material such asa glass frit, or a resin that is curable at room temperature such as atwo-component type resin, a light curable resin, a heat-curable resin,and the like can be used for the sealing film 518.

A partition wall 508 is provided so as to cover end portions of adjacentstacks of the first conductive film 506 and the second conductive film507. A structure 509 is provided over the partition wall 508. Note thatthe first conductive film 506 has a function as a reflective electrodeand a function as an anode of the light-emitting element. The secondconductive film 507 has a function of adjusting the optical path lengthof each light-emitting element. The EL layer 510 is formed over thesecond conductive film 507, and the third conductive film 512 is formedover the EL layer 510. The third conductive film 512 has a function as asemi-transmissive and semi-reflective electrode and a function as acathode of the light-emitting element. The structure 509 is providedbetween the light-emitting element and the coloring layer and has afunction as a spacer.

The EL layer 510 can be shared by the light-emitting elements (504R,504G, 504B, and 504W). Note that each of the light-emitting elements(504R, 504G, 504B, and 504W) has a micro optical resonator (ormicrocavity) structure which allows light emitted from the EL layer 510to resonate by the first conductive film 506 and the third conductivefilm 512; thus, spectra of light with different wavelengths can benarrowed and extracted even when they include the same EL layer 510.Specifically, by adjusting the thickness of each of the secondconductive films 507 provided under the EL layer 510 in thelight-emitting element (504R, 504G, 504B, or 504W), a desired emissionspectrum can be obtained from the EL layer 510, so that light emissionwith high color purity can be obtained. Therefore, the structureillustrated in FIG. 29A does not require a process of separately formingEL layers with different colors, and facilitates achieving highresolution.

The light-emitting device illustrated in FIG. 29A includes the coloringlayer (the color filter). Therefore, by using the microcavity structureand the color filter in combination, light emission with higher colorpurity can be obtained. Specifically, the optical path length of thelight-emitting element 504R is adjusted so that red light emission isprovided; red light is emitted in the direction indicated by an arrowthrough the coloring layer 514R. Furthermore, the optical path length ofthe light-emitting element 504G is adjusted so that green light emissionis provided; green light is emitted in the direction indicated by anarrow through the coloring layer 514G. Furthermore, the optical pathlength of the light-emitting element 504B is adjusted so that blue lightemission is provided; blue light is emitted in the direction indicatedby an arrow through the coloring layer 514B. Furthermore, the opticalpath length of the light-emitting element 504W is adjusted so that whitelight emission is provided; white light is emitted in the directionindicated by an arrow through the coloring layer 514W.

Note that a method for adjusting the optical path length of eachlight-emitting element is not limited thereto. For example, the opticalpath length may be adjusted by controlling the film thickness of the ELlayer 510 in each light-emitting element.

The coloring layers (514R, 514G, and 514B) may have a function oftransmitting light in a particular wavelength region. For example, a red(R) color filter for transmitting light in a red wavelength range, agreen (G) color filter for transmitting light in a green wavelengthrange, a blue (B) color filter for transmitting light in a bluewavelength range, or the like can be used. The coloring layer 514W maybe formed using an acrylic-based resin material which does not contain apigment or the like. The coloring layers (514R, 514G, 514B, and 514W)can be formed using any of various materials by a printing method, aninkjet method, an etching method using a photolithography technique, orthe like.

The first conductive film 506 can be formed using, for example, a metalfilm having high reflectivity (reflection factor of visible light is 40%to 100%, preferably 70% to 100%). The conductive film 506 can be formedusing a single layer or a stacked layer using aluminum, silver, or analloy containing such a metal material (e.g., an alloy of silver,palladium, and copper).

The second conductive film 507 can be formed using, for example,conductive metal oxide. As the conductive metal oxide, indium oxide, tinoxide, zinc oxide, indium tin oxide, indium zinc oxide, or any of thesemetal oxide materials in which silicon oxide or tungsten oxide iscontained can be used. Providing the second conductive film 507 ispreferable because the formation of an insulating film between the ELlayer 510 to be formed later and the first conductive film 506 can besuppressed. Furthermore, conductive metal oxide which is used as thesecond conductive film 507 may be formed in layer lower than the firstconductive film 506.

The third conductive film 512 is formed using a conductive materialhaving reflectivity and a conductive material having alight-transmitting property, and visible light reflectivity of the filmis preferably 20% to 80%, more preferably 40% to 70%. As the thirdconductive film 512, for example, silver, magnesium, an alloy of such ametal material, or the like is formed to be thin (e.g., 10 nm or less),and then, conductive metal oxide which can be used for the secondconductive film 507 is formed.

The above-described light-emitting device has a structure in which lightis extracted from the second substrate 516 side (a top emissionstructure), but may have a structure in which light is extracted fromthe first substrate 501 side where the FETs 500 are formed (a bottomemission structure) or a structure in which light is extracted from boththe first substrate 501 side and the second substrate 516 side (a dualemission structure). In the case of the bottom emission structure, thecoloring layers (514R, 514G, 514B, and 514W) may be formed under thefirst conductive film 506. Note that a light-transmitting substrate maybe used for the substrate through which light is transmitted, and alight-transmitting substrate and a light-blocking substrate may be usedfor the substrate through which light is not transmitted.

In FIG. 29A, the structure in which the light-emitting elements emitlight of red (R), green (G), blue (B), and white (W) is illustrated asan example. However, a structure is not limited thereto. For example, astructure in which the light-emitting elements emit light of red (R),green (G), and blue (B) may be used.

Here, a connection between the light-emitting element and the FET isdescribed in detail using FIG. 29B. Note that FIG. 29B is an example ofa structure of a region 520 surrounded by a dashed line shown in FIG.29A.

In FIG. 29B, an insulating film 522 functioning as a planarization filmis formed over the FET 500. Furthermore, an opening portion 524 reachinga conductive film functioning as a source electrode or a drain electrodeof the FET 500 is formed in the insulating film 522. Furthermore, thefirst conductive film 506 connected to the conductive film functioningas a source electrode or a drain electrode of the FET 500 is formed overthe insulating film 522. Furthermore, the second conductive film 507 isformed over the first conductive film 506.

The structure of the FET 500 is similar to the structure of thetransistor 100A described in the above embodiment; therefore,description thereof is omitted.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 6

In this embodiment, a configuration example of a display device of oneembodiment of the present invention is described.

<Configuration Example of Display Device>

FIG. 31A is a top view of a display device of one embodiment of thepresent invention. FIG. 31B illustrates a pixel circuit where a liquidcrystal element is used for a pixel of a display device of oneembodiment of the present invention. FIG. 31C illustrates a pixelcircuit where an organic EL element is used for a pixel of a displaydevice of one embodiment of the present invention.

Any of the above-described transistors can be used as a transistor usedfor the pixel. Here, an example in which an n-channel transistor is usedis shown. Note that a transistor manufactured through the same steps asthe transistor used for the pixel may be used for a driver circuit. Anyof the above-described capacitors can be used as a capacitor used forthe pixel. Thus, by using any of the above-described transistors andcapacitors for a pixel or a driver circuit, the display device can havehigh display quality and/or high reliability.

FIG. 31A illustrates an example of a top view of an active matrixdisplay device. A pixel portion 5001, a first scan line driver circuit5002, a second scan line driver circuit 5003, and a signal line drivercircuit 5004 are provided over a substrate 5000 in the display device.The pixel portion 5001 is electrically connected to the signal linedriver circuit 5004 through a plurality of signal lines and iselectrically connected to the first scan line driver circuit 5002 andthe second scan line driver circuit 5003 through a plurality of scanlines. Pixels including display elements are provided in respectiveregions divided by the scan lines and the signal lines. The substrate5000 of the display device is electrically connected to a timing controlcircuit (also referred to as a controller or a control IC) through aconnection portion such as an FPC.

The first scan line driver circuit 5002, the second scan line drivercircuit 5003, and the signal line driver circuit 5004 are formed overthe substrate 5000 where the pixel portion 5001 is formed. Therefore, adisplay device can be manufactured at cost lower than that in the casewhere a driver circuit is separately formed. Further, in the case wherea driver circuit is separately formed, the number of wiring connectionsis increased. By providing the driver circuit over the substrate 5000,the number of wiring connections can be reduced. Accordingly, thereliability and/or yield can be improved.

<(1) Liquid Crystal Display Device>

FIG. 31B illustrates an example of a circuit configuration of the pixel.Here, a pixel circuit which is applicable to a pixel of a VA liquidcrystal display device, or the like is illustrated.

This pixel circuit can be applied to a structure in which one pixelincludes a plurality of pixel electrodes. The pixel electrodes areconnected to different transistors, and the transistors can be drivenwith different gate signals. Accordingly, signals applied to individualpixel electrodes in a multi-domain pixel can be controlledindependently.

A gate wiring 5012 of a transistor 5016 and a gate wiring 5013 of atransistor 5017 are separated so that different gate signals can besupplied thereto. In contrast, a source or drain electrode 5014functioning as a data line is shared by the transistors 5016 and 5017.Any of the above-described transistors can be used as appropriate aseach of the transistors 5016 and 5017. Any of the above-describedcapacitors can be used as appropriate as each of the capacitors 5023 aand 5023 b. Thus, the liquid crystal display device can have highdisplay quality and/or high reliability.

A first pixel electrode is electrically connected to the transistor 5016and a second pixel electrode is electrically connected to the transistor5017. The first pixel electrode and the second pixel electrode areseparated. A shape of the first pixel electrode and the second pixelelectrode is not especially limited, for example, may be a V-like.

A gate electrode of the transistor 5016 is electrically connected to thegate wiring 5012, and a gate electrode of the transistor 5017 iselectrically connected to the gate wiring 5013. When different gatesignals are supplied to the gate wiring 5012 and the gate wiring 5013,operation timings of the transistor 5016 and the transistor 5017 can bevaried. As a result, alignment of liquid crystals can be controlled.

Furthermore, a capacitor may be formed using a capacitor wiring 5010, aninsulating film functioning as a dielectric, and a capacitor electrodeelectrically connected to the first pixel electrode or the second pixelelectrode.

The multi-domain pixel includes a first liquid crystal element 5018 anda second liquid crystal element 5019. The first liquid crystal element5018 includes the first pixel electrode, a counter electrode, and aliquid crystal layer therebetween. The second liquid crystal element5019 includes the second pixel electrode, a counter electrode, and aliquid crystal layer therebetween.

Note that a pixel circuit in the display device of one embodiment of thepresent invention is not limited to that shown in FIG. 31B. For example,a switch, a resistor, a capacitor, a transistor, a sensor, a logiccircuit, or the like may be added to the pixel illustrated in FIG. 31B.

<(2) Light-Emitting Device>

FIG. 31C illustrates another example of a circuit configuration of thepixel. Here, a pixel structure of a display device using alight-emitting element typified by an organic EL element (such a deviceis referred to as a light-emitting device) is described.

In an organic EL element, by application of voltage to a light-emittingelement, electrons are injected from one of a pair of electrodesincluded in the organic EL element and holes are injected from the otherof the pair of electrodes, into a layer containing a light-emittingorganic compound; thus, current flows. The electrons and holes arerecombined, and thus, the light-emitting organic compound is excited.The light-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Based on such a mechanism, such alight-emitting element is referred to as a current-excitation typelight-emitting element.

FIG. 31C illustrates an example of a pixel circuit. Here, an example inwhich two n-channel transistors and one capacitor are used in one pixelis illustrated. Note that any of the above-described transistors can beused as the n-channel transistors. Any of the above-described capacitorscan be used as the capacitor. Furthermore, digital time grayscaledriving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of apixel employing digital time grayscale driving will be described.

A pixel 5020 includes a switching transistor 5021, a driver transistor5022, a light-emitting element 5024, and a capacitor 5023. A gateelectrode of the switching transistor 5021 is connected to a scan line5026, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 5021 is connected to a signalline 5025, and a second electrode (the other of the source electrode andthe drain electrode) of the switching transistor 5021 is connected to agate electrode of the driver transistor 5022. The gate electrode of thedriver transistor 5022 is connected to a power supply line 5027 throughthe capacitor 5023, a first electrode of the driver transistor 5022 isconnected to the power supply line 5027, and a second electrode of thedriver transistor 5022 is connected to a first electrode (a pixelelectrode) of the light-emitting element 5024. A second electrode of thelight-emitting element 5024 corresponds to a common electrode 5028. Thecommon electrode 5028 is electrically connected to a common potentialline provided over the same substrate.

As each of the switching transistor 5021 and the driver transistor 5022,any of the above-described transistors can be used. Any of theabove-described capacitors can be used as the capacitor 5023. In thismanner, an organic EL display device having high display quality and/orhigh reliability can be provided.

The potential of the second electrode (the common electrode 5028) of thelight-emitting element 5024 is set to be a low power supply potential.Note that the low power supply potential is lower than a high powersupply potential supplied to the power supply line 5027. For example,the low power supply potential can be GND, 0 V, or the like. The highpower supply potential and the low power supply potential are set to behigher than or equal to the forward threshold voltage of thelight-emitting element 5024, and the difference between the potentialsis applied to the light-emitting element 5024, whereby current issupplied to the light-emitting element 5024, leading to light emission.The forward voltage of the light-emitting element 5024 refers to avoltage at which a desired luminance is obtained, and includes at leastforward threshold voltage.

Note that gate capacitance of the driver transistor 5022 may be used asa substitute for the capacitor 5023 in some cases, so that the capacitor5023 can be omitted. The gate capacitance of the driver transistor 5022may be formed between the channel formation region and the gateelectrode.

Next, a signal input to the driver transistor 5022 is described. In thecase of a voltage-input voltage driving method, a video signal forturning on or off the driver transistor 5022 is input to the drivertransistor 5022. In order for the driver transistor 5022 to operate in alinear region, voltage higher than the voltage of the power supply line5027 is applied to the gate electrode of the driver transistor 5022.Note that voltage higher than or equal to voltage which is the sum ofpower supply line voltage and the threshold voltage Vth of the drivertransistor 5022 is applied to the signal line 5025.

In the case of performing analog grayscale driving, a voltage higherthan or equal to a voltage which is the sum of the forward voltage ofthe light-emitting element 5024 and the threshold voltage Vth of thedriver transistor 5022 is applied to the gate electrode of the drivertransistor 5022. A video signal by which the driver transistor 5022 isoperated in a saturation region is input, so that current is supplied tothe light-emitting element 5024. In order for the driver transistor 5022to operate in a saturation region, the potential of the power supplyline 5027 is set higher than the gate potential of the driver transistor5022. When an analog video signal is used, it is possible to supplycurrent to the light-emitting element 5024 in accordance with the videosignal and perform analog grayscale driving.

Note that in the display device of one embodiment of the presentinvention, a pixel configuration is not limited to that shown in FIG.31C. For example, a switch, a resistor, a capacitor, a sensor, atransistor, a logic circuit, or the like may be added to the pixelcircuit shown in FIG. 31C.

For example, FIG. 32A illustrates an applicable example of a pixelcircuit. Here, an example in which three n-channel transistors and onecapacitor are used in one pixel is illustrated.

FIG. 32A illustrates an example of a circuit diagram of a pixel 5111.The pixel 5111 includes a transistor 5155, a transistor 5156, atransistor 5157, a capacitor 5158, and a light-emitting element 5154.

The potential of a pixel electrode in the light-emitting element 5154 iscontrolled in accordance with an image signal Sig input to the pixel5111. The luminance of the light-emitting element 5154 depends on apotential difference between the pixel electrode and the commonelectrode.

The transistor 5156 has a function of controlling electrical connectionbetween a wiring SL and a gate of the transistor 5155. One of the sourceand the drain of the transistor 5155 is electrically connected to ananode of the light-emitting element 5154, and the other of the sourceand the drain is electrically connected to a wiring VL. The transistor5157 has a function of controlling electrical connection between awiring ML and the other of the source and the drain of the transistor5155. One of a pair of electrodes of the capacitor 5158 is electricallyconnected to the gate of the transistor 5155, and the other iselectrically connected to the anode of the light-emitting element 5154.

The switching of the transistor 5156 is performed in accordance with thepotential of a wiring GL which is electrically connected to a gate ofthe transistor 5156. The switching of the transistor 5157 is performedin accordance with the potential of the wiring GL which is electricallyconnected to a gate of the transistor 5157.

Note that any of the above-described transistors can be used for atleast one of the transistors 5155, 5156, and 5157. Furthermore, any ofthe above-described capacitors can be used for the capacitor 5158.

For example, any of the following expressions can be used for the casewhere a source (or a first terminal or the like) of a transistor iselectrically connected to X through (or not through) Z1 and a drain (ora second terminal or the like) of the transistor is electricallyconnected to Y through (or not through) Z2, or the case where a source(or a first terminal or the like) of a transistor is directly connectedto one part of Z1 and another part of Z1 is directly connected to Xwhile a drain (or a second terminal or the like) of the transistor isdirectly connected to one part of Z2 and another part of Z2 is directlyconnected to Y.

Examples of the expressions include, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit structure is defined byan expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope. Note that one embodiment of the present invention isnot limited to these expressions which are just examples. Here, each ofX, Y, Z1, and Z2 denotes an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, a layer,or the like).

Next, an operation example of the pixel 5111 illustrated in FIG. 32A isdescribed.

FIG. 32B shows an example of a timing chart of the potentials of thewiring GL electrically connected to the pixel 5111 in FIG. 32A and thepotential of the image signal Sig supplied to the wiring SL. Note thatin the timing chart in FIG. 32B, all the transistors included in thepixel 5111 in FIG. 32A are n-channel transistors.

First, in a period t1, a high-level potential is applied to the wiringGL. Accordingly, the transistor 5156 and the transistor 5157 are turnedon. A potential Vdata of the image signal Sig is applied to the wiringSL, and the potential Vdata is applied to the gate of the transistor5155 through the transistor 5156.

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL. The potential Vano is preferably higher thanthe sum of the potential Vcat, the threshold voltage Vthe of thelight-emitting element 5154, and the threshold voltage Vth of thetransistor 5155. The above potential difference is provided between thewiring VL and the wiring CL, so that the value of the drain current ofthe transistor 5155 is determined by the potential Vdata. Then, thedrain current is supplied to the light-emitting element 5154, wherebythe luminance of the light-emitting element 5154 is determined.

In the case where the transistor 5155 is an n-channel type, it ispreferable that, in the period t1, the potential of the wiring ML belower than the sum of the potential of the wiring CL and the thresholdvoltage Vthe of the light-emitting element 5154, and the potential ofthe wiring VL be higher than the sum of the potential of the wiring MLand the threshold voltage Vth of the transistor 5155. With the aboveconfiguration, the drain current of the transistor 5155 can be made toflow preferentially through the wiring ML instead of the light-emittingelement 5154 even when the transistor 5157 is on.

Next, in a period t2, a low-level potential is applied to the wiring GL.Accordingly, the transistor 5156 and the transistor 5157 are turned off.When the transistor 5156 is off, the potential Vdata is held at the gateof the transistor 5155. A potential Vano is applied to the wiring VL,and a potential Vcat is applied to the wiring CL. Thus, thelight-emitting element 5154 emits light in accordance with the luminancedetermined in the period t1.

Next, in a period t3, a high-level potential is applied to the wiringGL. Accordingly, the transistor 5156 and the transistor 5157 are turnedon. In addition, such a potential that the gate voltage of thetransistor 5155 is higher than the threshold voltage Vth thereof isapplied to the wiring SL. The potential Vcat is applied to the wiringCL. Then, the potential of the wiring ML is lower than the sum of thepotential of the wiring CL and the threshold voltage Vthe of thelight-emitting element 5154, and the potential of the wiring VL ishigher than the sum of the potential of the wiring ML and the thresholdvoltage Vth of the transistor 5155. With the above configuration, thedrain current of the transistor 5155 can be made to flow preferentiallythrough the wiring ML instead of the light-emitting element 5154.

Then, the drain current of the transistor 5155 is supplied to a monitorcircuit through the wiring ML. The monitor circuit generates a signalincluding information about the value of the drain current by using thedrain current flowing through the wiring ML. Thus, using the abovesignal, the light-emitting device according to one embodiment of thepresent invention can correct the value of the potential Vdata of theimage signal Sig supplied to the pixel 5111.

Note that in the light-emitting device including the pixel 5111illustrated in FIG. 32A, the operation in the period t3 is notnecessarily performed after the operation in the period t. For example,in the pixel 5111, the operation in the period t3 may be performed afterthe operations in the periods t1 and t2 are repeated a plurality oftimes. Alternatively, after the operation in the period t3 is performedon pixels 5111 in one row, the light-emitting elements 5154 may bebrought into a non-light-emitting state by writing an image signalcorresponding to the lowest grayscale level 0 to the pixels 5111 in therow which have been subjected to the above operation. Then, theoperation in the period t3 may be performed on pixels 5111 in the nextrow.

Alternatively, a configuration of a pixel circuit illustrated in FIG.33A may be employed. FIG. 33A illustrates an example of a pixel circuit.Here, an example in which four n-channel transistors and one capacitorare used in one pixel is illustrated.

A pixel 5211 illustrated in FIG. 33A includes a transistor 5215, atransistor 5216, a transistor 5217, a capacitor 5218, a light-emittingelement 5214, and a transistor 5219.

The potential of a pixel electrode in the light-emitting element 5214 iscontrolled in accordance with an image signal Sig input to the pixel5211. The luminance of the light-emitting element 5214 depends on apotential difference between the pixel electrode and the commonelectrode.

The transistor 5219 has a function of controlling electrical connectionbetween the wiring SL and a gate of the transistor 5215. One of a sourceand a drain of the transistor 5215 is electrically connected to an anodeof the light-emitting element 5214. The transistor 5216 has a functionof controlling electrical connection between the wiring VL and the otherof the source and the drain of the transistor 5215. The transistor 5217has a function of controlling electrical connection between the wiringML and the other of the source and the drain of the transistor 5215. Oneof a pair of electrodes of the capacitor 5218 is electrically connectedto the gate of the transistor 5215, and the other is electricallyconnected to the anode of the light-emitting element 5214.

The switching of the transistor 5219 is performed in accordance with thepotential of a wiring GLa which is electrically connected to a gate ofthe transistor 5219. The switching of the transistor 5216 is performedin accordance with the potential of a wiring GLb which is electricallyconnected to a gate of the transistor 5216. The switching of thetransistor 5217 is performed in accordance with the potential of awiring GLc which is electrically connected to a gate of the transistor5217.

Note that any of the above-described transistors can be used for atleast one of the transistor 5215, the transistor 5216, the transistor5217, and the transistor 5219. Furthermore, any of the above-describedcapacitors can be used for the capacitor 5218.

Next, an example of operation of the pixel 5211 illustrated in FIG. 33Afor external correction is described.

FIG. 33B shows an example of a timing chart of potentials of the wiringGLa, the wiring GLb, and the wiring GLc, which are electricallyconnected to the pixel 5211 illustrated in FIG. 33A, and a potential ofthe image signal Sig supplied to the wiring SL. Note that the timingchart of FIG. 33B is an example in which all the transistors included inthe pixel 5211 shown in FIG. 33A are n-channel transistors.

First, in a period t1, a high-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, and alow-level potential is applied to the wiring GLc. Accordingly, thetransistors 5219 and 5216 are turned on and the transistor 5217 isturned off. A potential Vdata of the image signal Sig is applied to thewiring SL, and the potential Vdata is applied to the gate of thetransistor 5215 through the transistor 5219.

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL. The potential Vano is preferably higher thanthe sum of the potential Vcat and the threshold voltage Vthe of thelight-emitting element 5214. The potential Vano of the wiring VL isapplied to the other of the source and the drain of the transistor 5215through the transistor 5216. Thus, the value of the drain current of thetransistor 5215 is determined in accordance with the potential Vdata.Then, the drain current is supplied to the light-emitting element 5214,whereby the luminance of the light-emitting element 5214 is determined.

Next, in a period t2, a low-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, and alow-level potential is applied to the wiring GLc. Accordingly, thetransistor 5216 is turned on and the transistors 5219 and 5217 areturned off. Since the transistor 5219 is turned off, the potential Vdatais held at the gate of the transistor 5215. The potential Vano isapplied to the wiring VL, and the potential Vcat is applied to thewiring CL. Thus, the light-emitting element 5214 maintains the luminancedetermined in the period t1.

Next, in a period t3, a low-level potential is applied to the wiringGLa, a low-level potential is applied to the wiring GLb, and ahigh-level potential is applied to the wiring GLc. Accordingly, thetransistor 5217 is turned on and the transistors 5219 and 5216 areturned off. The potential Vcat is applied to the wiring CL. Thepotential Vano is applied to the wiring ML, which is connected to themonitor circuit.

By the above operation, the drain current of the transistor 5215 issupplied to the light-emitting element 5214 through the transistor 5217.In addition, the drain current is also supplied to the monitor circuitthrough the wiring ML. The monitor circuit generates a signal includinginformation about the value of the drain current by using the draincurrent flowing through the wiring ML. Thus, using the above signal, thelight-emitting device according to one embodiment of the presentinvention can correct the value of the potential Vdata of the imagesignal Sig supplied to the pixel 5211.

Note that in the light-emitting device including the pixel 5211illustrated in FIG. 33A, the operation in the period t3 is notnecessarily performed after the operation in the period t2. For example,in the light-emitting device, the operation in the period t3 may beperformed after the operations in the periods t1 and t2 are repeated aplurality of times. Alternatively, after the operation in the period t3is performed on pixels 5211 in one row, the light-emitting elements 5214may be brought into a non-light-emitting state by writing an imagesignal corresponding to the lowest grayscale level 0 to the pixels 5211in the row which have been subjected to the above operation. Then, theoperation in the period t3 may be performed on pixels 5211 in the nextrow.

Alternatively, a configuration of a pixel circuit illustrated in FIG.34A may be employed. FIG. 34A illustrates an example of a pixel circuit.Here, an example in which five n-channel transistors and one capacitorare used in one pixel is illustrated.

A pixel 5311 illustrated in FIG. 34A includes a transistor 5315, atransistor 5316, a transistor 5317, a capacitor 5318, a light-emittingelement 5314, a transistor 5319, and a transistor 5320.

The transistor 5320 has a function of controlling electrical connectionbetween a wiring RL and the anode of the light-emitting element 5314.The transistor 5319 has a function of controlling electrical connectionbetween the wiring SL and a gate of the transistor 5315. One of a sourceand a drain of the transistor 5315 is electrically connected to an anodeof the light-emitting element 5314. The transistor 5316 has a functionof controlling electrical connection between the wiring VL and the otherof the source and the drain of the transistor 5315. The transistor 5317has a function of controlling electrical connection between the wiringML and the other of the source and the drain of the transistor 5315. Oneof a pair of electrodes of the capacitor 5318 is electrically connectedto the gate of the transistor 5315, and the other is electricallyconnected to the anode of the light-emitting element 5314.

The switching of the transistor 5319 is performed in accordance with thepotential of the wiring GLa which is electrically connected to a gate ofthe transistor 5319. The switching of the transistor 5316 is performedin accordance with the potential of the wiring GLb which is electricallyconnected to a gate of the transistor 5316. The switching of thetransistor 5317 is performed in accordance with the potential of thewiring GLc which is electrically connected to a gate of the transistor5317. The switching of the transistor 5320 is performed in accordancewith the potential of the wiring GLd which is electrically connected toa gate of the transistor 5320.

Note that any of the above-described transistors can be used for atleast one of the transistor 5315, the transistor 5316, the transistor5317, the transistor 5319, and the transistor 5320. Furthermore, any ofthe above-described capacitors can be used for the capacitor 5318.

Next, an example of operation of the pixel 5311 illustrated in FIG. 34Afor external correction is described.

FIG. 34B shows an example of a timing chart of potentials of the wiringGLa, the wiring GLb, the wiring GLc, and the wiring GLd, which areelectrically connected to the pixel 5311 illustrated in FIG. 34A, and apotential of the image signal Sig supplied to the wiring SL. Note thatin the timing chart in FIG. 34B, all the transistors included in thepixel 5311 in FIG. 34A are n-channel transistors.

First, in a period t1, a high-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, a low-levelpotential is applied to the wiring GLc, and a high-level potential isapplied to the wiring GLd. Accordingly, the transistors 5319, 5316, and5320 are turned on and the transistor 5317 is turned off. A potentialVdata of the image signal Sig is applied to the wiring SL, and thepotential Vdata is applied to the gate of the transistor 5315 throughthe transistor 5319. Thus, the value of the drain current of thetransistor 5315 is determined by the potential Vdata. A potential Vanois applied to the wiring VL and a potential V1 is applied to the wiringRL; therefore, the drain current flows between the wiring VL and thewiring RL through the transistor 5316 and the transistor 5320.

The potential Vano is preferably higher than the sum of the potentialVcat and the threshold voltage Vthe of the light-emitting element 5314.The potential Vano of the wiring VL is applied to the other of thesource and the drain of the transistor 5315 through the transistor 5316.The potential V1 applied to the wiring RL is applied to the one of thesource and the drain of the transistor 5315 through the transistor 5320.The potential Vcat is applied to the wiring CL.

Note that it is preferable that the potential V1 be sufficiently lowerthan a potential obtained by subtracting the threshold voltage Vth ofthe transistor 5315 from the potential V0. The light-emitting element5314 does not emit light in the period t1 because the potential V1 canbe set sufficiently lower than the potential obtained by subtracting thethreshold voltage Vthe of the light-emitting element 5314 from thepotential Vcat.

Next, in a period t2, a low-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, a low-levelpotential is applied to the wiring GLc, and a low-level potential isapplied to the wiring GLd. Accordingly, the transistor 5316 is turned onand the transistors 5319, 5317, and 5320 are turned off. Since thetransistor 5319 is off, the potential Vdata is held at the gate of thetransistor 5315.

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL. Accordingly, the drain current of thetransistor 5315, the value of which is determined in the period t1, issupplied to the light-emitting element 5314 because the transistor 5320is turned off. By supply of the drain current to the light-emittingelement 5314, the luminance of the light-emitting element 5314 isdetermined, and the luminance is held in the period t2.

Next, in a period t3, a low-level potential is applied to the wiringGLa, a low-level potential is applied to the wiring GLb, a high-levelpotential is applied to the wiring GLc, and a low-level potential isapplied to the wiring GLd. Accordingly, the transistor 5317 is turned onand the transistors 5319, 5316, and 5320 are turned off. The potentialVcat is applied to the wiring CL. The potential Vano is applied to thewiring ML, which is connected to the monitor circuit.

By the above operation, the drain current of the transistor 5315 issupplied to the light-emitting element 5314 through the transistor 5317.In addition, the drain current is also supplied to the monitor circuitthrough the wiring ML. The monitor circuit generates a signal includinginformation about the value of the drain current by using the draincurrent flowing through the wiring ML. Thus, using the above signal, thelight-emitting device according to one embodiment of the presentinvention can correct the value of the potential Vdata of the imagesignal Sig supplied to the pixel 5311.

Note that in the light-emitting device including the pixel 5311illustrated in FIG. 34A, the operation in the period t3 is notnecessarily performed after the operation in the period t2. For example,in the light-emitting device, the operation in the period t3 may beperformed after the operations in the periods t1 and t2 are repeated aplurality of times. Alternatively, after the operation in the period t3is performed on pixels 5311 in one row, the light-emitting elements 5314may be brought into a non-light-emitting state by writing an imagesignal corresponding to the lowest grayscale level 0 to the pixels 5311in the row which have been subjected to the above operation. Then, theoperation in the period t3 may be performed on pixels 5311 in the nextrow.

In the pixel 5311 illustrated in FIG. 34A, even when variation inresistance of a portion between the anode and the cathode of thelight-emitting element 5314 among pixels is caused by deterioration ofthe light-emitting element 5314 or the like, the potential of the sourceof the transistor 5315 can be set to a predetermined potential V1 at thetime of applying the potential Vdata to the gate of the transistor 5315.Thus, variation in luminance of the light-emitting element 5314 amongpixels can be prevented.

Alternatively, a configuration of a pixel circuit illustrated in FIG.35A may be employed. FIG. 35A illustrates an example of a pixel circuit.Here, an example in which six n-channel transistors and one capacitorare used in one pixel is illustrated.

A pixel 5411 illustrated in FIG. 35A includes a transistor 5415, atransistor 5416, a transistor 5417, a capacitor 5418, a light-emittingelement 5414, a transistor 5440, a transistor 5441, and a transistor5442.

The potential of a pixel electrode in the light-emitting element 5414 iscontrolled in accordance with an image signal Sig input to the pixel5411. The luminance of the light emitting element 5414 depends on apotential difference between the pixel electrode and the commonelectrode.

The transistor 5440 has a function of controlling electrical connectionbetween the wiring SL and one of a pair of electrodes of the capacitor5418. The other of the pair of electrodes of the capacitor 5418 iselectrically connected to one of a source and a drain of the transistor5415. The transistor 5416 has a function of controlling electricalconnection between the wiring VL1 and a gate of the transistor 5415. Thetransistor 5441 has a function of controlling electrical connectionbetween one of the pair of electrodes of the capacitor 5418 and the gateof the transistor 5415. The transistor 5442 has a function ofcontrolling electrical connection between one of the source and thedrain of the transistor 5415 and an anode of the light-emitting element5414. The transistor 5417 has a function of controlling electricalconnection between the other of the source and the drain of thetransistor 5415 and the wiring ML.

In FIG. 35A, the other of the source and the drain of the transistor5415 is electrically connected to the wiring VL.

The transistor 5440 is switched in accordance with the potential of thewiring GLa which is electrically connected to a gate of the transistor5440. The transistor 5416 is switched in accordance with the potentialof the wiring GLa which is electrically connected to a gate of thetransistor 5416. The transistor 5441 is switched in accordance with thepotential of the wiring GLb which is electrically connected to a gate ofthe transistor 5441. The transistor 5442 is switched in accordance withthe potential of the wiring GLb which is electrically connected to agate of the transistor 5442. The transistor 5417 is switched inaccordance with the potential of the wiring GLc which is electricallyconnected to a gate of the transistor 5417.

FIG. 35B shows an example of a timing chart of potentials of the wiringGLa, the wiring GLb, and the wiring GLc, which are electricallyconnected to the pixel 5411 illustrated in FIG. 35A, and a potential ofthe image signal Sig supplied to the wiring SL. Note that in the timingchart in FIG. 35B, all the transistors included in the pixel 5411 inFIG. 35A are n-channel transistors.

First, in a period t1, a low-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, and ahigh-level potential is applied to the wiring GLc. Accordingly, thetransistors 5441, 5442, and 5417 are turned on, and the transistors 5440and 5416 are turned off. The transistors 5442 and 5417 are turned on,whereby a potential V0, which is the potential of the wiring ML, isapplied to the one of the source and the drain of the transistor 5415and the other of the pair of electrodes of the capacitor 5418(represented as a node A).

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL. The potential Vano is preferably higher thanthe sum of the potential V0 and the threshold voltage Vthe of thelight-emitting element 5414. Note that the potential V0 is preferablylower than the sum of the potential Vcat and the threshold voltage Vtheof the light-emitting element 5414. With the potential V0 set in theabove range, current can be prevented from flowing through thelight-emitting element 5414 in the period t1.

A low-level potential is then applied to the wiring GLb, and thetransistors 5441 and 5442 are accordingly turned off and the node A isheld at the potential V0.

Next, in a period t2, a high-level potential is applied to the wiringGLa, a low-level potential is applied to the wiring GLb, and a low-levelpotential is applied to the wiring GLc. Accordingly, the transistors5440 and 5416 are turned on, and the transistors 5441, 5442, and 5417are turned off.

In the transition from the period t1 to the period t2, it is preferablethat the potential applied to the wiring GLa be changed from low to highand then the potential applied to the wiring GLc be changed from high tolow. This operation prevents change in the potential of the node A dueto the change of the potential applied to the wiring GLa.

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL. A potential Vdata of the image signal Sig isapplied to the wiring SL, and a potential V1 is applied to the wiringVL1. Note that the potential V1 is preferably higher than the sum of thepotential Vcat and the threshold voltage Vth of the transistor 5415 andlower than the sum of the potential Vano and the threshold voltage Vthof the transistor 5415.

Note that in the pixel structure shown in FIG. 35A, even if thepotential V1 is higher than the sum of the potential Vcat and thethreshold voltage Vthe of the light-emitting element 5414, thelight-emitting element 5414 does not emit light as long as thetransistor 5442 is off. Thus, the allowable potential V0 range can beexpanded and the allowable range of V1−V0 can also be increased. As aresult of increasing the degree of freedom of values for V1−V0, thethreshold voltage Vth of the transistor 5415 can be obtained accuratelyeven when time required to obtain the threshold voltage Vth of thetransistor 5415 is reduced or limited.

By this operation, the potential V1 which is higher than the sum of thepotential of the node A and the threshold voltage Vth is input to thegate of the transistor 5415 (represented as a node B), and thetransistor 5415 is turned on. Thus, electric charge in the capacitor5418 is discharged through the transistor 5415, and the potential of thenode A, which is the potential V0, starts to increase. The potential ofthe node A finally converges to the potential V1−Vth and the gatevoltage of the transistor 5415 converges to the threshold voltage Vth ofthe transistor 5415; then, the transistor 5415 is turned off.

The potential Vdata of the image signal Sig applied to the wiring SL isapplied to the one of the pair of electrodes of the capacitor 5418(represented as a node C) through the transistor 5440.

Next, in a period t3, a low-level potential is applied to the wiringGLa, a high-level potential is applied to the wiring GLb, and alow-level potential is applied to the wiring GLc. Accordingly, thetransistors 5441 and 5442 are turned on, and the transistors 5440, 5416,and 5417 are turned off.

In the transition from the period t2 to the period t3, it is preferablethat the potential applied to the wiring GLa be changed from high to lowand then the potential applied to the wiring GLb be changed from low tohigh. This structure can prevent potential change of the node A due tochange of the potential applied to the wiring GLa.

A potential Vano is applied to the wiring VL, and a potential Vcat isapplied to the wiring CL.

The potential Vdata is applied to the node B by the above operation;thus, the gate voltage of the transistor 5415 becomes Vdata−V1+Vth.Thus, the gate voltage of the transistor 5415 can be the value to whichthe threshold voltage Vth is added. With this structure, variation inthe threshold voltage Vth of the transistor 5415 can be reduced. Thus,variation of current values supplied to the light-emitting element 5414can be suppressed, whereby reducing unevenness in luminance of thelight-emitting device.

Note that the potential applied to the wiring GLb is greatly variedhere, whereby an influence of variation of threshold voltages of thetransistor 5442 on the value of a current supplied to the light-emittingelement 5414 can be prevented. In other words, the high-level potentialapplied to the wiring GLb is much higher than the threshold voltage ofthe transistor 5442, and the low-level potential applied to the wiringGLb is much lower than the threshold voltage of the transistor 5442;thus, on/off switching of the transistor 5442 is secured and theinfluence of variation of threshold voltages of the transistor 5442 onthe value of current supplied to the light-emitting element 5414 can beprevented.

Next, in a period t4, a low-level potential is applied to the wiringGLa, a low-level potential is applied to the wiring GLb, and ahigh-level potential is applied to the wiring GLc. Accordingly, thetransistor 5417 is turned on and the transistors 5416, 5440, 5441, and5442 are turned off.

In addition, a potential Vano is applied to the wiring VL, and thewiring ML is electrically connected to the monitor circuit.

By the above operation, drain current Id of the transistor 5415 flowsnot to the light-emitting element 5414 but to the wiring ML through thetransistor 5417. The monitor circuit generates a signal includinginformation about the value of the drain current Id by using the draincurrent Id flowing through the wiring ML. The magnitude of the draincurrent Id depends on the mobility or the size (channel length, channelwidth) of the transistor 5415. Using the above signal, thelight-emitting device according to one embodiment of the presentinvention can thus correct the value of the potential Vdata of the imagesignal Sig supplied to the pixel 5411. That is, the influence ofvariation in the mobility of the transistor 5415 can be reduced.

Note that in the light-emitting device including the pixel 5411illustrated in FIG. 35A, the operation in the period t4 is notnecessarily always performed after the operation in the period t3. Forexample, in the light-emitting device, the operation in the period t4may be performed after the operations in the periods t1 to t3 arerepeated a plurality of times. Alternatively, after the operation in theperiod t4 is performed on pixels 5411 in one row, the light-emittingelements 5414 may be brought into a non-light-emitting state by writingan image signal corresponding to the lowest grayscale level 0 to thepixels 5411 in the row which have been subjected to the above operation.Then, the operation in the period t4 may be performed on pixels 5411 inthe next row.

Note that, in the light-emitting device including the pixel 5411illustrated in FIG. 35A, the other of the source and the drain of thetransistor 5415 is electrically isolated from the gate of the transistor5415, so that their potentials can be individually controlled.Accordingly, in the period t2, the potential of the other of the sourceand the drain of the transistor 5415 can be set higher than a potentialobtained by adding the threshold voltage Vth to the gate potential ofthe transistor 5415. Thus, when the transistor 5415 is normally on, thatis, when the threshold voltage Vth is negative, charge can beaccumulated in the capacitor 5418 until the source potential of thetransistor 5415 becomes higher than the gate potential V1 of thetransistor 5415. For these reasons, in the light-emitting deviceaccording to one embodiment of the present invention, even when thetransistor 5415 is a normally on transistor, the threshold voltage Vthcan be obtained in the period t2; and in the period t3, the gate voltageof the transistor 5415 can be set to a value obtained by adding thethreshold voltage Vth.

Therefore, in the light-emitting device of one embodiment of the presentinvention, display unevenness can be reduced and high-quality images canbe displayed even if the transistor 5415 becomes a normally-ontransistor.

Not only the characteristics of the transistor 5415 but also thecharacteristics of the light-emitting element 5414 may be monitored.Here, it is preferable that current not flow through the transistor 5415by controlling the potential Vdata of the image signal Sig, for example.The current of the light-emitting element 5414 can be thus extracted,and degradation or variation in current characteristics of thelight-emitting element 5414 can be obtained.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 7

In this embodiment, a display module and electronic appliances that canbe formed using a semiconductor device of one embodiment of the presentinvention are described with reference to FIG. 36 and FIGS. 37A to 37H.

In a display module 8000 illustrated in FIG. 36, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight unit 8007 includes a light source 8008. Note that althougha structure in which the light sources 8008 are provided over thebacklight unit 8007 is illustrated in FIG. 36, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which the light source 8008 is provided at an end portionof the backlight unit 8007 and a light diffusion plate is furtherprovided may be employed. Note that the backlight unit 8007 need not beprovided in the case where a self-luminous light-emitting element suchas an organic EL element is used or in the case where a reflective panelor the like is employed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 37A to 37H illustrate electronic appliances. These electronicappliances can include a housing 9000, a display portion 9001, a speaker9003, an LED lamp 9004, operation keys 9005 (including a power switch oran operation switch), a connection terminal 9006, a sensor 9007 (asensor having a function of measuring or sensing force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredray), a microphone 9008, and the like.

FIG. 37A illustrates a mobile computer that can include a switch 9009,an infrared port 9010, and the like in addition to the above components.FIG. 37B illustrates a portable image reproducing device (e.g., a DVDplayer) that is provided with a memory medium and can include a seconddisplay portion 9002, a memory medium reading portion 9011, and the likein addition to the above components. FIG. 37C illustrates a goggle-typedisplay that can include the second display portion 9002, a support9012, an earphone 9013, and the like in addition to the abovecomponents. FIG. 37D illustrates a portable game machine that caninclude the memory medium reading portion 9011 and the like in additionto the above components. FIG. 37E illustrates a digital camera that hasa television reception function and can include an antenna 9014, ashutter button 9015, an image receiving portion 9016, and the like inaddition to the above components. FIG. 37F illustrates a portable gamemachine that can include the second display portion 9002, the memorymedium reading portion 9011, and the like in addition to the abovecomponents. FIG. 37G illustrates a television receiver that can includea tuner, an image processing portion, and the like in addition to theabove components. FIG. 37H illustrates a portable television receiverthat can include a charger 9017 capable of transmitting and receivingsignals, and the like in addition to the above components.

The electronic appliances illustrated in FIGS. 37A to 37H can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Furthermore, the electronic appliance including aplurality of display portions can have a function of displaying imagedata mainly on one display portion while displaying text data on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images on a plurality of display portions with a parallaxtaken into account, or the like. Furthermore, the electronic applianceincluding an image receiving portion can have a function of shooting astill image, a function of taking a moving image, a function ofautomatically or manually correcting a shot image, a function of storinga shot image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying a shotimage on the display portion, or the like. Note that functions that canbe provided for the electronic appliances illustrated in FIGS. 37A to37H are not limited to those described above, and the electronicappliances can have a variety of functions.

The electronic appliances described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic appliance that does not have a displayportion.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

EXAMPLE

In this example, a cross-sectional shape of a transistor of oneembodiment of the present invention was observed.

A method for manufacturing a sample which was observed in this examplewill be described below. Note that in this example, a transistorcorresponding to the transistor 100 illustrated in FIGS. 1A and 1C wasmanufactured.

First, the substrate 102 was prepared. As the substrate 102, a glasssubstrate was used. Next, as the insulating film 108 a, a 100-nm-thicksilicon nitride film (SiN-1) was formed over the substrate 102. Next, asthe insulating film 108 b, a 400-nm-thick silicon oxynitride film(SiON-1) was formed over the insulating film 10 a. Note that theinsulating film 108 a and the insulating film 108 b were successivelyformed in a vacuum using a PECVD apparatus.

S Next, as the film that suppresses release of oxygen, a 5-nm-thicktantalum nitride film was formed over the insulating film 108 b. Notethat the tantalum nitride film was formed using a sputtering apparatus.Next, oxygen was added to the insulating film 108 b from the tantalumnitride film side using an ashing apparatus. Next, the tantalum nitridefilm was removed using a dry etching apparatus.

Next, as the oxide semiconductor film 110, a 50-nm-thick oxidesemiconductor film (IGZO) was formed over the insulating film 108 b.Note that a sputtering apparatus was used for forming the oxidesemiconductor film 110; a metal oxide of In:Ga:Zn=1:1:1.2 [atomic %] wasused as a sputtering target, and an AC power supply was used forsupplying power to the sputtering target. Next, heat treatment wasperformed on the substrate over which the oxide semiconductor film 110was formed. As the heat treatment, heat treatment under a nitrogenatmosphere at a temperature of 450° C. for one hour and heat treatmentunder a mixed gas of nitrogen and oxygen at a temperature of 450° C. forone hour were sequentially performed.

Next, a mask was formed over the oxide semiconductor film 110 by alithography step, and the oxide semiconductor film 110 was processedinto an island-like shape using the mask. Note that the oxidesemiconductor film 110 was processed by a wet etching method using achemical solution.

Next, as the insulating film 112, a 100-nm-thick silicon oxynitride film(SiON-2) was formed over the island-shaped oxide semiconductor film 110.Note that the insulating film 112 was formed using a PECVD apparatus.

Next, as the conductive film 114 a, a 30-nm-thick tantalum nitride film(TaN) was formed over the insulating film 112. Next, as the conductivefilm 114 b, a 150-nm-thick tungsten film (W) was formed over theconductive film 114 a. Note that the conductive film 114 a and theconductive film 114 b were successively formed in a vacuum using asputtering apparatus.

Next, a mask was formed over the conductive film 114 b by a lithographystep, and the conductive films 114 b and 114 a and the insulating film112 were processed into an island-like shape using the mask. Theprocessing of the conductive films 114 a and 114 b and the insulatingfilm 112 was performed using a dry etching apparatus. Next, the impurityelement was added to the oxide semiconductor film 110 with the maskleft. The impurity element was added as follows. An etching apparatuswas used, the substrate was placed between parallel plates in a chamberof the etching apparatus, and then, an argon gas was introduced to thechamber, and an RF power was applied between the parallel plates so thata bias was applied on the substrate side.

Next, as the insulating film 118, a 100-nm-thick silicon nitride film(SiN-2) was formed to cover the insulating film 108 b, the oxidesemiconductor film 110, the insulating film 112, and the conductivefilms 114 a and 114 b. Next, as the insulating film 120, a 300-nm-thicksilicon oxynitride film (SiON-3) was formed over the insulating film118. Note that the insulating film 118 and the insulating film 120 weresuccessively formed in a vacuum using a PECVD apparatus.

Next, a mask was formed over the insulating film 120 by a lithographystep, and opening portions were formed in the insulating films 120 and118 using the mask. Note that the opening portions reach the oxidesemiconductor film 110. The opening portions were formed using a dryetching apparatus.

Next, conductive films were formed to cover the insulating film 120 andthe opening portion. As the conductive films, a 50-nm-thick tungstenfilm, a 400-nm-thick aluminum film, and a 100-nm-thick titanium filmwere stacked in this order. Note that the conductive films weresuccessively formed in a vacuum using a sputtering apparatus.

Next, a mask was formed over the conductive film by a lithography step,and the conductive film 122 and the conductive film 124 were formedusing the mask.

Through the above-described process, the sample for cross-sectionalobservation of this example was manufactured.

FIGS. 38A and 38B show the results of cross-sectional observation. Notethat a transmission electron microscope (TEM) was used for thecross-sectional observation.

FIG. 38A is a cross-sectional TEM image of the vicinity of theconductive film 114 in the dashed-dotted line X1-X2 direction shown inFIG. 1A. FIG. 38B is a cross-sectional TEM image of the vicinity of theconductive film 114 in the dashed-dotted line Y1-Y2 direction shown inFIG. 1A.

Note that “SiN-1”, “SiN-2”, “SiON-1”, “SiON-2”, “SiON-3”, “TaN”, and “W”in FIGS. 38A and 38B correspond to film kinds described in the aboveparentheses in Example. Furthermore, “Pt” in FIGS. 38A and 38B denotesplatinum of surface coating for cross-sectional observation.

It is found from the cross-sectional TEM image shown in FIG. 38A that anend portion of the tantalum nitride film (TaN) is positioned on theouter side than an end portion of the tungsten film (W). Furthermore, anend portion of the silicon oxynitride film (SiON-2) is positioned on theouter side than the end portion of the tantalum nitride film (TaN). Itis found from the cross-sectional TEM image shown in FIG. 38B that anend portion of the tantalum nitride film (TaN) is positioned on theouter side than an end portion of the tungsten film (W). Furthermore, anend portion of the silicon oxynitride film (SiON-2) is positioned on theouter side than the end portion of the tantalum nitride film (TaN).Furthermore, the silicon oxynitride film (SiON-1) has a depressedportion in a region that does not overlap with the silicon oxynitridefilm (SiON-2). It is found from the cross-sectional TEM images shown inFIGS. 38A and 38B that, in the sample formed in this example, thesilicon nitride film (SiN-2) has high coverage and a favorablecross-sectional shape.

The structure described in this example can be used in appropriatecombination with any of the structures described in the embodiments.

This application is based on Japanese Patent Application serial no.2014-020517 filed with Japan Patent Office on Feb. 5, 2014, and JapanesePatent Application serial no. 2014-037209 filed with Japan Patent Officeon Feb. 27, 2014, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A semiconductor device comprising: a firsttransistor, a second transistor, a third transistor, a capacitor, alight-emitting element, a first wiring, a second wiring, a third wiring,and a fourth wiring, wherein a gate electrode of the first transistor iselectrically connected to one of a source electrode and a drainelectrode of the second transistor, and one terminal of the capacitor,wherein one of a source electrode and a drain electrode of the firsttransistor is electrically connected to one of a source electrode and adrain electrode of the third transistor, the other terminal of thecapacitor, and the light-emitting element, wherein the other of thesource electrode and the drain electrode of the first transistor iselectrically connected to the first wiring, wherein the other of thesource electrode and the drain electrode of the second transistor iselectrically connected to the second wiring, wherein a gate electrode ofthe second transistor is electrically connected to the third wiring,wherein the other of the source electrode and the drain electrode of thethird transistor is electrically connected to the fourth wiring, whereinthe first transistor includes: an oxide semiconductor film over a firstinsulating film; a gate insulating film over the oxide semiconductorfilm; a gate electrode over the gate insulating film; a secondinsulating film over the gate electrode; a source electrode electricallyconnected to the oxide semiconductor film through a first opening of thesecond insulating film; and a drain electrode electrically connected tothe oxide semiconductor film through a second opening of the secondinsulating film, wherein the second insulating film includes a region incontact with a top surface of the gate insulating film, and a region incontact with a side surface of the gate insulating film, wherein thefirst insulating film includes a first region and a second region,wherein the first region overlaps with the oxide semiconductor film,wherein the second region does not overlap with the oxide semiconductorfilm, does not overlap with the gate insulating film, and does notoverlap with the gate electrode, wherein a film thickness of the firstinsulating film in the second region is smaller than a film thickness ofthe first insulating film in the first region.
 2. A semiconductor devicecomprising: a first transistor, a second transistor, a third transistor,a capacitor, a light-emitting element, a first wiring, a second wiring,a third wiring, and a fourth wiring, wherein a gate electrode of thefirst transistor is electrically connected to one of a source electrodeand a drain electrode of the second transistor and one terminal of thecapacitor, wherein one of a source electrode and a drain electrode ofthe first transistor is electrically connected to one of a sourceelectrode and a drain electrode of the third transistor, the otherterminal of the capacitor, and the light-emitting element, wherein theother of the source electrode and the drain electrode of the firsttransistor is electrically connected to the first wiring, wherein theother of the source electrode and the drain electrode of the secondtransistor is electrically connected to the second wiring, wherein agate electrode of the second transistor is electrically connected to thethird wiring, wherein the other of the source electrode and the drainelectrode of the third transistor is electrically connected to thefourth wiring, wherein the first transistor includes: a first conductivefilm; a first insulating film over the first conductive film; an oxidesemiconductor film over the first insulating film; a gate insulatingfilm over the oxide semiconductor film; a gate electrode over the gateinsulating film; a second insulating film over the gate electrode; asource electrode electrically connected to the oxide semiconductor filmthrough a first opening of the second insulating film; and a drainelectrode electrically connected to the oxide semiconductor film througha second opening of the second insulating film, wherein the firstconductive film includes a region overlapping with the oxidesemiconductor film through the first insulating film, wherein the secondinsulating film includes a region in contact with a top surface of thegate insulating film, and a region in contact with a side surface of thegate insulating film, wherein the first insulating film includes a firstregion and a second region, wherein the first region overlaps with theoxide semiconductor film, wherein the second region does not overlapwith the oxide semiconductor film, does not overlap with the gateinsulating film, and does not overlap with the gate electrode, wherein afilm thickness of the first insulating film in the second region issmaller than a film thickness of the first insulating film in the firstregion.